It swells like a golf ball when he is about to come. This can be very useful information because some people go slightly soft just before they come, and you might tend erroneously to back off. You must be very careful not to give someone more anal stimulation than they want. Some guys want none at all. Some want you just to rest a finger on the outside.
Some guys come from touching the prostate alone.
Generally, the looser the anus the more likely it is the guy enjoys comcomitant anal stimulation. Anal stimulation is a whole other essay. Since giving the perfect blow job takes much trial and error, you will do your best work if you have the same partner time after time, whom you get to know inside and out. However, there are several ways to deal with this. Don't push the penis so far into your throat that it blocks off the air.