Stretch it sweetie! Your mouth and tongue are muscles. You may feel silly but just start playing with your mouth and tongue more, it's the easiest first step, be aware that you have it, move it around more, make silly face while you're driving in the car alone. This will activate the muscles and make them slowly stronger.
Then, don't be afraid to add in your hand and give that jaw and neck a break. You can make a blow job sexier with extras, it isn't just about your mouth. That's just one of the components when blowing him away
Mostly though, experiment! Find new positions and things that work for you. Ask him what he wants different. Listen, and don't take it personally. The more your experiment, the more perfect you'll customize his oral experience!