It is perfectly normal for penis to be thin at base and fat at top. Your boyfriend does not have any kind of problem or weaknesses so do not be anxious and have safe sex with him.
Shift your focus from the penis appearance and invest your energy to make strong relationship so you both can have good future together. Masturbation has nothing to do with his penis structure hence do not nag him if he masturbates. Masturbation is a healthy way to remove sexual urge and safe too.