It's similar action of holding your urine during pee-ing, but of course you don't hold your urine to exercise it as it's not healthy to do so.

you bf can try it to hold his urine during pee-ing and catch that motion/ feeling. After that try it after finish his business. The same motion/ exercise, (hold it 5 sec, relax 5 sec), (hold it 5 sec, relax 5 sec) 10 times 3 set each session. after few times, he may feel vibrate/ sore on his muscle and its normal due to muscle weak/ tired.

try wear underwear with his foreskin pulled back thru out the day or during sleeping. It feels uncomfortable, that's part of making the inner flesh less sensitive

Btw, did your bf wash his penis by pulling back the foreskin? if he did, the inner skin/ flesh should be less sensitive thru out the years.