This.
Always love him and be there for him.
Teach him to be a good person, regardless of religion.

and remember the parable of the prodigal son.

just be there for him.

I think that in the end you need to most concern yourself with the fact that he is family and that he is still very young. Be there for him on that level and let him find his own path to discovery.

It would be really horrible for both parties if religious pressure were to cause damage between the relationship.

at 13 years old, your young brother have to experience life like anybody else and having douts regarding faith its part of it.