Since no one has given you an actual reply and it sounds like you have permission I'm happy to give some simple advice.
Most stallions that are halter broke don't mind having their balls touched. Start out slow and simple with lots of petting. Be careful of biting since most stallions love to bite things. Most also like belly scratches and butt scratches. Go slow and just move your hands around minding his reactions.
Generally stallions don't really get sexually excited by ball fondling. They might like the rubbing and such because they can't scratch there. They also generally aren't big on hand jobs unless taught how to accept them. Penis touching is usually ok as long as he has given you permission to touch under his belly. Again just mind his reactions and don't get kicked. Ball fondling will come quickly and easily once he likes you.