If your wife is using, and using the family's funds for drugs and neglecting S13, then I would gather SOLID evidence and I would see a "men's rights" family law attorney pronto, and move for temporary full custody of my kid(s).
Your wife is obviously under the influence of two VERY strong addictions -- her affair, and drugs. Someone in this state cannot be assumed to do the right things for themselves, or even their own children. Add to that (as if that formula isn't potent enough already), the physical abuse stuff??? WARNING, WILL ROBINSON!!!
PROTECT YOURSELF . . . and your son.
Starsky
It's powerful forces in play here, and having been on both sides of it I would take Starsky309's warning very seriously. These are powerful forces which grow stronger over time as they gain foothold in your W's mind.