Since he was bat**** crazy (he attended a "special needs" school and was reported to have been in the Army for one month before being discharged) there is a decent chance that he was collecting SSI. Living with mom, that could have all been spending money.
So my guess is that he got the money for the 13 guns from you (and me).
(And kudos to dad for all the interest he took in raising his son! I think most normal dads would know at least a little bit about their son's shooting hobby.)
Search engine: Parental Alienation
Youtube: search on Dr. Craig Childress