I can taste the precum, but its not a good sign he is about to cum, a lot of guys make precum long before they cum. But, like everyone else said, he should tell you when he's about to cum, otherwise he's never getting a blowjob again.
Agreed ahead of time that he will signal you before he comes. Any boy worth being with will abide… especially if he ever wants another blowjob from you.
Talk to your partner. Explain you're not comfortable taking a mouthful and ask him to tap your head when he's close. Work out where it's going ahead of time like a napkin, cup or floor.
On the flip side cup and grip his falls. You'll feel him twitch down there as he gets close and goes over. Takes practice to figure out the grip and his spots.