From your description it sounds like you have more than one problem with your a/c system. The guy recharged it, installed dye and oil, and it was blowing cold, albeit not as cold as you thought it should. If you had a problem with the systems performance the time to bring that up would've been when he approached you. A short time after the repair the system quit working suddenly and then started working again after a breif cool down period. This description is not indicative of a system that has lost it's charge but is more symptomatic of a system that has a control issue like a faulty relay, cycling switch, clutch, compressor, control head, etc. After the tech did his repairs to your car the system was working acceptably, at this point he has no reason to go any further on your car as it is fixed and is showing no further signs of failure and you have not given him any information to the contrary. He is a mechanic and not a psychic so he cannot predict the future and know that you will have other issues with the same system and he cannot induce a fault into the system so that he could repair it. In short, if the system has other unforseen problems he is going to have to wait for the to duplicate so he can fix them.
Please understand that this guy could be the best tech out there or an absolute moron and a criminal too boot but it sounds like there was not a lot he could do about the situation that you have with your a/c. Being a tech and a shop owner I have been in this guys shoes and I know that it sucks but let me ask you this, if it turns out that you need a $1,000.00+ compressor how would you have felt if he told you this at the time and said "it appears to be working now but it might go out so I recommend that you buy one today". If you have any sense at all you would've clutched your wallet and backed away slowly while asking him to total the bill so you could take your car as is. In the end the guy can't read the future and can only repair the problem as it presents itself to him. The guy cannot diagnose what will not duplicate in his shop.

Mark