If your snake has just recently started to stop eating this isn't a cause for concern. Boas can go up to 7 months of not eating without any serious effects, especially during the winter months or during breeding season for females. Keep offering food and possibly switch to live food to interest him more.
It could just be hybernating,try turning the temp up a couple of degrees,and persistance,it once took me about 25 mins of wiggling a mouse to get my python to eat
