Are there any tips or tricks to get my bp to gain weight?

Ok so long story short my sister had a 8 yr old male bp. I noticed he wasn't being taken care of properly so I offered to take him. So I've had him for 2.5 months now. He had no heat source just newspaper water and a hide. Hadn't been handled in probably a year. Surprisingly he's not aggressive and after a few times of handling him he does not ball up as soon as you touch him anymore. She had been feeding him every 2 weeks a small f/t rat. But once he got to my house he didn't eat for 6 weeks. And the only way I got him to eat is offer adult mice. I know there not big enough for him but he refuses to eat rats now. He eats 1 every 4 days. When I weighed him he only weighs 177 grams. Which is extremely under weight,right? This is my first snake he's so sweet and I just want to get him nice and healthy again since he was neglected for a long time. Any help would be greatly appreciated.

Was 177 grams a typo?

Yes sorry I meant 766 grams.

That's still extremely small. He must have been severely underfed his whole life. I don't know how old my biggest bp is (I'd always assumed maybe 5, but he could be 15 for all I know) but even directly from the poor conditions I rescued him from he was fully grown and weighed at least 1600 grams. I didn't have a chance to weigh him for the first couple of months, so I can't tell for sure. At 766 grams your guy should be able to manage a small rat with no problem. There are some snakes that really do refuse rats and only eat mice, but I'd keep trying the rats. My youngest came to me from the breeder eating live small rats, but when I tried to feed her live she always ran from it. I ended up feeding her a live mouse, then switched to f/t small rats. She never strikes it, but will always eat it if I lay it in her tub. To me it almost seems like she's afraid of her food, I don't know why.

Yea we learned that early on he will only eat if I lay it in front of him and leave the room for a while. I don't know a lot about them but I've read that they sometimes get hooked on mice or rats and because he was fed f/t rats before no problem I think he just decided he likes mice better right now.

A small rat every 2 weeks won't make a ball python anywhere near that small at 8 years. That snake is either no more than 2 years, or he was not being fed a small rat every two weeks. At 8 years and that size, that's more like a small mouse every two months... one small rat every two weeks is not too bad a schedule. At 8 years, if the snake were healthy and fed well it would only need one medium every 2-3 weeks

Let him eat mice until he's at a better size if that's what he'll eat. I wouldn't do one every four days though, I'd do 2 once a week. So he has time to digest. Once he's up to a reasonable size then you can try switching him

I couldn't believe his size either. But I know he is 8 yrs old cause she bought it for her son in '09. She had no heat on him and had a respiratory infection when I brought him home which we have treated.

Yeah feed him every weekend, 2 large/jumbo mice for a few months. Get him up to 1500G and then you can play with the food source ect.. Id just get him healthy for now.

Ok thank you guys!