The clue that he's cuming is when you see him tightly grip her buttocks and he is looking closely at her pussy and anus. Once his pleasure is over he releases his tight animal grip.
I love how he pulls her bum cheeks apart to look at her lips, clit and anus and lick them. It makes him so excited. Then he holds her buttock cheeks tightly as he feels his little cock go rigid with pleasure and his "love cream" pumps into her mouth.