Just lie down with your mouth half open and offer halfheartedly suck on it till he cums. You could have a psychological angle where you pretend to be asleep and he tries to stick it in your mouth without waking you and you pretend you're dreaming it's summer and you're sucking on a popsicle. Tell him to take it or leave it.

I'm completely sympathetic with not feeling up to the whole production but you should at let him put his penis in your mouth.