You could try positive reinforcement. Reward no cursing--for example, every day that he only says nice words, give him a sticker or a lollipop or whatever he would consider a reward. After a week, let him have a bigger treat, like a toy. The other thing you could do is tell him that it was "cooler and more of a big boy thing" to say things like "oh fudge" or "oh sugar". If he hears it from you he will definitely want to copy it! Best of luck!
On another note, financial constraints and no "adult" time can be very difficult and trying for anyone. Perhaps there is something you could do from home for a few hours a week (e.g., telemarketing or sales) that would provide an outlet for you and some extra "fun" money. If you were able to do this in the afternoon, you could have a teen-babysitter for a few hours which is not too expensive, particularly if you are in the house so no emergencies will need to be dealt with by a babysitter.