I had an adult male pastel go 6 months with no ill effects. This guy was kept in the same conditions as 40 other snakes that ate fine, he bred like a champ but not interested in food. Eventually took live rat pups then small rats again.

I would try to keep his temps up, 84 cool 94 warm side and that should help get his metabolism going.

If he is not loosing weight I would'nt worry too much, he will eat eventually.