It swells like a golf ball when he is about to come. The excitation state of the prostate can be very useful information because some penises go slightly soft just before they come, and you might be tempted to back off. You must be 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. Quite often guys who refuse anal stimulation on the grounds it is not manly or it is dirty will go absolutely wild if you attempt it just prior to orgasm. On the other hand, you could totally put him off. It is a bit of a gamble. 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.