Sucking on it like sexually, or like a baby, breastfeeding?
Either way, there's no harm to you. Sexually, you might enjoy it, but tell him if you like or don't like what he's doing.
If you are currently lactating, and he wants to try the milk, that's okay, so long as your baby is getting what it needs. If your breasts are tender/sore, tell him, and make sure he understands he needs to listen to your feedback on it.