Lion79--
The foreskin can be too tight and still retract from the head when erect. If it is having trouble sliding back when having sex and is red and swollen after pulling out, then phimosis is probably indicated. But there are varying degrees of phimosis.
Find out from him if he has any trouble with the foreskin when flaccid or masturbating and, if he doesn't, then he probably does need to see a urologist. They do have creams that he can apply which will stretch the skin or he may need a circumcision depending on the diagnosis.
That's why a doctor needs to see what course of treatment would be the best. Good luck.