Thanks for your question.
Sensual massage is low-risk for STIs.
Massage with a “happy ending” can mean different things, but often it means that the person receiving the massage also receives sexual stimulation until they ejaculate (cum). The “happy ending” can be done by hand (“hand job”) or mouth (oral sex or “blow job”).
If the “happy ending” is done by hand, then it is very low-risk for STIs. If the “happy ending” was done by mouth (without a condom), then there is a risk for STIs, including: Chlamydia, Gonorrhea, Syphilis, Herpes and HPV. A person can have these STIs without having any symptoms, and the only way to know if they have an STI is to get tested. If the “happy ending” is done by mouth with the condom on, then it is very-low risk for STIs.
If you’re able to speak with your partner more about how he receives the “happy ending” you will have more ideas about potential STI risks.
Hope this helps. Please feel free to comment below or submit another question as needed.