I think it was right after he graduated college and as he was working for Wendy's. Given the school issues Chris had when he was younger, his parents might have been able to dupe themselves into believing the "discrimination" was happening again, however the day that Chris came home from Wendy's, his uniform soaked with trash, and himself absolutely reeking of fast food garbage, with Chris walking around in public like that, really cemented his mentality and lack of capability. Even if he lied to them about being fired for scaring a small child, it was probably right then that Bob knew Chris wasn't going to go into a drafting job, and it was better for the Chandlers that they just kept Chris away from the public as much as they could.
His mother had to drill it into his head that he shouldn't be telling people he's a "virgin with rage" all the time, which meant he was telling a lot of people. Then there was having to try to get him to stop with the attraction sign and the mess that all caused.
I firmly believe that Chris's homework through high school and college was done by his father, but once he was out in the real world, there was no way to cheat to make him look normal, and mainstreaming absolutely did not help.