D.D.
Big mistake to move in with your SO due to financial issues. Moving in where someone has already established it as his home means you will be lesser than in the relationship. Add in a teenager and it'll be very stressful.
You need to figure it out on your own. Cut back on as many expenses as you can. Move to a smaller place, get a second or 3rd job to bring in the money lost from child support and alimony. Have your son get a job and use that money to pay bills. Set the example that yes things can get hard but you have nobody but yourself to dig yourself out of a hole.