I am owed over 40k in back child support for my child. Father has just been approved for SSD and my child will get monthly benefits as well. Normally CSE intercepts tax refunds, etc. and they have already sent an income withholding order to SSA. My question is: Will they intercept his retro lump sum payment for the child support arrearage?

Make sure they know the lump sum is coming and you might be able to get a goodly portion of it provided they do the paperwork in time.