Perceived medical wisdom says that babies shouldn't get chickenpox if breastfed.
So much so that when my DC1 did get the pox at 4mo, the health visitor and GP didn't believe me, even when I took her to the GP the first time and showed him the spots!
They believed me the following morning, when it was very obvious!
Anyway, we got off lightly, she had a mild case, we taped mittens onto her and she hasn't had it since (now almost 4yo) in spite of all her friends at nursery having had it since. The immunity to not having it again isn't guaranteed if they have it under 12 months, supposedly.
I think your cousin is being selfish for taking his DS when there will be older people there and also babies.
Having got off lightly with DC1 I would probably myself run the risk with DC2 rather than risk missing such a major family event. But I genuinely don't know what to advise you other than trying to get your cousin to back down and not take his DS.
I have found that many parents are a bit more relaxed about these things with DC2 - I caught tonsilitis from a play date with DC1's friend, for example, and was quite hacked off about it. But the mum of the friend, for whom this is DC2, didn't see an issue at all and has been sending her DD to Pre-school as normal, as well as letting her climb all over me and my baby!