There is no risk for syphilis from kissing, mutual masturbation, and fingering.
When you say frotting - this depends. Assuming you were both unclothed, your genitals would have to come into direct contact with a syphilis sore in order to get it. Do you know if he had a syphilis sore? Was he diagnosed because of symptoms or with a blood test?
Do you have a sore?
If you do not have a sore yet, you are not contagious. This is called the incubation period (the time from infection to symptoms), and syphilis is not infectious during this time. Other STIs are infectious during the incubation period, but syphilis isn't.
If he doesn't know if he had a sore, you should be treated, just in case. The sore is painless, so it's easy to miss if you aren't looking for it.