no,
your baby can get hbv because he is playing ( even your baby didin take vaccination).
The baby is fine....use precautions...no open cuts around the baby, no sharing drinks, the usual. Don't deprive your brother of the love he needs from your child and don't make him feel like he is a freak...he's not; and the baby will be fine.
how about if he kiss the baby lips to lips?
Check that your baby has developed antibodies. If he has, he is safe. If not, he may need a bootser or re-vaccination.