If you have any milk at all, try by putting just maybe 1/4 formula in a bottle with 3/4 breast milk. Work your way up to all formula. Also, if he is not used to a bottle, try putting it in a sippy cup or even a regular cup which you hold for him (my son completely skipped sippy cups and was drinking from a regular cup--with my help--at 8 months. He just didnt want the sippy)
I had this w/my daughter, and although it's not the most pleasant way to get him to drink formula, I will tell you that he will do it, even if you have no milk left. I got pregnant while nursing and although my midwife said I could keep nursing, my body did not agree and my milk supply compltely dried up. 100% gone. My daughter had never had formula and she wanted no part of it. She went a couple of days starving and screaming b/c she wouldn't take the formula. I felt awful, but could do nothing about it! By about day 3, she was slurping down the formula (I'm sure she was SO hungry!). My daughter was younger than your son and so was not yet on solid foods. If he's getting lots of solids, he may not be as anxious to go to formula, but if he's hungry enough, his basic instincts will kick in and he'll drink formula. I'd suggest making sure he's very hungry (don't give any solids beforehand) when you try to formula. Obviously I'm not suggesting not giving solids--just to do so after you've tried him to take the formula!
ps--I see several people are saying to keep nursing and it will come back. I've nursed my children with no formula at all, but with my daughter, it did NOT come back. My body was obviously tired with the other pregnancy and just did not want to produce any more milk! I pumped, saw a lactation consultant many times, did the whole drip-milk on the nipple thing, fenugreek, etc. I was absolutely dead set against formula. Although it's true MOST times you can get the milk supply back up, do not beat yourself up about it or feel guilty if you can't.