When should I be worried?

A month ago I rescued a pair of hatchling champagnes from a bad situation. I was told that they had 4-5 meals, and were a month or so old. The female refused a live fuzzy mouse, and picky rat the first week but had no issue taking the f/t the three last weeks. The male how ever was a bit smaller then her to start with, probably 70g at most. He also refused both the live and frozen. Its now been a month-month and a half since his last meal. All his conditions are good, Ive had no issues with my other 21 snakes, just him.
My question is, when do I try to assist feed/force feed?

If he looks good and not too malnourished I would give him another week. Pre-scent his cage with shavings then leave a live rat pup or small mouse hopper in his cage overnight. You don't want to wait too long, though, or else even assist feeding might be too much for his system to handle.