As far as I know, the greatest risk of contracting STIs is if you have an open sore in your mouth, or if you have an ulcer, though I'm sure there are definite possibilities in catching an STI without any of the above listed exceptions.

first check penis for small cuts check to see if there is any blood around
some std,s are hard to detect like herpe,s or genitle wart,s which can be transmitted by mouth, so untill you know him quite well make him wear a condom