Catching a disease from a handjob is exceptionaly unlikely. If he had blood or semen on his hand then it might be possible but you would notice that and simple hand washing rules that out.
With oral sex it's comparitively easy to pass on gonorrea and chlamydia, but then these infections are not dangerous and are easily cured. HIV transmission is possible from a blow job but rare. There was a time when some men used condoms for oral sex, that habit has gone away now because it takes a lot of the fun out of the sucking. It hardly seems worth it when the risks are so small.
I have seen men wash their cocks with medicated soap immediately after sex, how much good that habit does I never found out.
When you look at the balance of odds, barebacking is so very much more risky than any other kind of sex that by comparison wanking and sucking give very little cause for worry and a great deal of enjoyment.