My son is being abused by his wife.

[Captain Picard]

OP, I want to elaborate a little bit on why I indicated I thought you might need to consider that you son could be the abusive party. This may not be the case, but you need to consider this as you consider getting social service or outside parties involved. Here is some information on wounds that appear when someone is trying defend themself. You mentioned marks on his arms and that raised a HUGE red flag for me.

Self-defense wounds
i. Aggressor may have scratches on
arms and hands or bite marks on
the chest or arm
ii. Victim may have bruises on
forearms, scratches on his/her own
neck, bilateral injuries, etc.

Where did you get this information ~~ its seems it may be a generalization of a male/attacker v. female/victim scenario but not a female/attacker v. male/victim