Additional reasons he could be growling is that he really want to tie and is expressing sexual frustration when he slips out, or he could be oversensitive and feeling discomfort when he slips out.
If you haven't already, try testing how he would do while tied by jacking him off, getting him fully erect, and holding his knot in your hand for 5 minutes at least. You can do that once while unmounted because it's easier and then try again next time he's horny by doing it while mounted. Hold his knot right by your pleasure hole so he has to maneuver and go butt to butt with you as he normally would when tied with a dog. Then tell us the results of that testing.
If he is feeling discomfort or frustration when pulling out prematurely, then the solution is to try to keep him tied. The testing will hopefully give you the confidence to do that during sex. It sounds like you're a woman so you'd need to use your hand to hold him behind the knot to stay inside you. Some dogs will not get fully hard without continuous pressure behind the knot supplied by your hand. A canine bitch supplies that pressure but a human vagina does not.