OP, bless you, you sound sweet although a bit naive! I work in sexual health with young people, and I can tell you without a doubt, there is every possibility that he is doing more than just kissing!
What you need to tell him:
-the condom is the ONLY way to protect against STIs and pregnancy. Even if she is using contraception, they should also always use a condom
-once he starts having sex, he should have regular sti screens every 6 months
-sti's can be passed on through oral sex, vaginal sex, anal sex, toy sharing, and mutual masturbation (touching self and touching her then touching self...)
-many sti's have no symptoms at all, Chlamydia is especially common, and usually has no symptoms, so one should test regularly for Chlamydia
-sexually transmitted infections and parasites can be treated and cured (chlamydia, public lice, crabs, gonnorhoea, syphillis)
-sexually transmitted diseases can be treated for symptoms, but cannot be cured, meaning that you will always carry the virus even though you may have periods without symptoms (warts, herpes, HIV)
-you need to use a condom for oral sex also, as STIs can be transmitted that way
-if you live in London he can sign up for a c-card and get free condoms up to the age of 25
-www.swish.org is a great website