What are possible reasons that my boyfriend's penis has red small spots on the tip and burning after sex?

There is: No way to tell over the internet, because we can't see the problem. Have his doctor take a look at it while the problem is still visible.

Here are some...: Likely, he has some long foreskin-related inflammation, which may be temporarily cared with topical antifungal with steroid cream as instructed after cleansing and drying so to keep up adequate hygiene. For long-term care, circumcision is most advisable. Nonetheless, see pro-evaluation & counseling so to decide the needed scope of care timely.