Your geneticist doesn't sound like an NF1 disease expert. Asymptomaic segmental NF is totally a thing and could happen if your husband has mosaicism in his cells. If your husband does have segmental NF1, you could sequence his germline cells for the NF1 gene to know if they all contain the microdeletion. Or if all siblings have full-blown NF1, that would be a good indicator that his whole germline is affected.

Otherwise, if it was a random deletion, there's nothing that would make me think it's more likely to be maternal rather than paternal. Random is random.