Hi rr8483, firstly, let me just welcome you to the board, secondly, you are right, Jehovahs Witnesses, even those not baptised are encouraged not to have a relationship with a non JW, however, if he was practising and really practising I'm sure you would know as he would have little spare time and it would just be obvious.
My advice is talk to him about it, maybe hes just attending the odd meeting to keep family members happy.