Kissing very rarely, if ever, transmits syphilis - that's more of a theoretical risk than a practical one, unless the person has a syphilis sore on their lips.
Kissing or sucking a nipple is highly unlikely to transmit syphilis. First, the nipple isn't a mucus membrane like the mouth or genitals, and unless he had an active syphilis sore on his tongue, he wouldn't transmit it.
Syphilis isn't that common in most developed countries, so this isn't something I'd spend any time worrying about.