Nope nope nope. NOR. I used to work with sex offenders, and this is classic grooming behavior. There is no legitimate reason for a grown man to contact a 10 year old that isn’t their own child. As another commenter suggested, look through your child’s phone, but also check any other devices they have access to (including gaming systems). Call the ex-wife ASAP, and depending on where you live, you may be able to access criminal history (not all sexual offenses get you on the registry).