I have engaged in various types of sexual activity with neutered male dogs over the past 50+ years, mostly in my mis-spent youth. They were castrated at ages ranging anywhere from 8 WEEKS to 14 years. The majority were done at between 6 months and 3 years. I had sex with some of them both "before" and "after."
I've found that nearly all neutered boys have some amount of interest and ability. It doesn't seem to make too much difference how long ago it was done, a bit more with how old they were when they were cut. If they had some sort of previous experience, that makes a HUGE difference! Most will hump at least a little, knot up and orgasm. Neutered dogs normally don't stay knotted more than a few minutes, but many are ready for another go after a short rest.
Interest varies. Some will hump like there's no tomorrow and later try to get you to do it again. Others are like, "Yeah, sure. Stroke it, suck it, whatever. Wake me when you're done." I've had a very few "Touch me there again and you're gonna die!" types. Some have been a bit skittish at first, but then kind of got into it later. Some were eager and humpy at first, but once they knotted up and started to orgasm, it seemed to frighten them and they ran off. I have experienced all of the above with intact dogs as well.
There is one common denominator: I have never seen a dog who has been neutered for more than a month or so ejaculate. A very small number would produce a few small droplets or maybe one quick squirt, that's it. I have read many times that they can and do ejaculate, but have only rarely observed it, even with dogs who were snipped only days earlier.
Two of my own (neutered) dogs tended to ejaculate streams of pee when they came, especially if they had a full bladder. One was on some rather strong medication for seizures, and the other was quite elderly. The volume of urine was similar to the amount of semen one might expect from an intact dog of similar size.
Of all the castrated dogs in my experience, there were perhaps 3 who seemed to be totally impotent. Two of those were neutered a small pups and one was done around 8-10 years. However, I had caught all of that group (individually) "off their turf," which might have something to do with their lack of performance. I have had encounters with intact males in the same circumstances with similar results.
The few who I had been active with "before" were still quite willing and eager and would often initiate sex. One dog, though, who I had never touched previously, got quite excited when I met him in the woods. He rode my leg a bit and tried mounting me when I got down on the ground with him. He was pretty well knotted in his sheath when I explored him a bit and gleefully face-fucked me several times. During another session, he became on of the very few dogs who I ever had mount and tie me. This boy was snipped at about a year when he was pulled from the pound and was probably 8 or so when he joined me on that hike. As a rescued dog, it's anybody's guess what experiences he might have had in his past life, perhaps he'd been with a Human before.
One of my own secondhand dogs that I adopted as an adult had been neutered as a small pup and had a pencil dick. Even so, he would get knotty during a belly rub. With some experimentation, I found that he loved oral. He would hump briskly, have a strong orgasm and hold a knot for 10 minutes or so. After the first few times, he would frequently initiate sex play and try to "herd" me to our special spot when he was in the mood. His penis would get much thicker when erect and had a clean, mild, somewhat creamy taste to it. He was very pleasant to suck. We did this many hundreds of times in the 10 years I had him. I lost him to hemangio-sarcoma, an aggressive cancer that attacks the blood vessels on the major organs. This seems to be a tale often told with early-neutered male dogs.
There were a few (both intact and neutered) who were uninterested, unwilling or seemed frightened by the whole thing. I respected that and left them alone. My current dog (yes, a neutered rescue) has a nice, big, suckable penis, but he only permits me to touch it on rare occasions. A session with him usually consists of a hand job and MAYBE a quick taste. He is a mix of several Northern breeds, I've read they can be like that.
Sex with neutered male dogs is basically a crapshoot. Many love it, a few hate it, still others are just a bit skittish or they're just indifferent. You just have to feel them out (or maybe feel them up?) Some who say "No!" at first, might come to enjoy it later. Or not. Sometimes you'll encounter one with "morning wood," or rolling in a sunny spot, feeling good about himself and showing off that fact with a cute little knot and a bit of pink, or overly excited while greeting you. Give him a little squeeze around and behind that knot and see what happens. If he's cool with it, enjoy. If he seems scared or annoyed, stop. Maybe try again another time. Neutered boys might mount you, but more likely won't. The can't provide you with a mouthful of cum, but I enjoy sucking them anyway. Figure out what they like and what they're capable of, and you just might be able to work around that empty sack.
Play nice, be gentle, treat him with love and respect whatever the outcome.
That is new. 
