It's great to hear your RI snakes are coming around and responding to treatment.

Interesting situation with your Fire. At 600 grams he's certainly big enough to have the girlies on his mind. Are you keeping him in close proximity to any females of breeding age? If you are, you could try moving in further away from them. I understand moving him might not be an option depending your set up, but I learned this trick from a breeder a few years back who swore by it.

I would only consider force feeding as a drastic last option, and I would try assist feeding well before I got to that point.