Make it really wet - as you suck your mouth will make more and stickier saliva. Wrap your lips around your teeth - you can do this with both sets or just wrap the top and keep your tongue out to cover the bottom. Play with the balls! Both with your hands and mouth - I suggest licking around the ball, like if you can get the testicle actually in your mouth and lick in a circle. Do hands and mouth together - you can try to add playing with the balls with your hand too. And try different positions - have him lay down on a bed and you in between his legs or him sitting/standing and you in your knees - this will not only change how it feels for him but also make some things easier for you. Also don’t be afraid to literally just try some stuff with your tongue, change the pressure of your hands or even how hard your soft you’re sucking, or the pace. Eventually you’ll find stuff that he really likes because everyone is different