i probably would not give more than one raising a week. there's always a discussion on here about raisins and other 'unhealthy' (i know, raisins sound healthy to me! but i'm a human and not a chinchilla
) treats. there are people who would not give them at all, mine get it very occasionally, a couple times a year perhaps. sweet treats like raisins can cause health problems like gas build up - my rescue chin nearly died of this and consequential stasis although it had nothing to do with treats.
does he ever get rolled oats or rosehips? these are good healthy treats. some people give shredded wheat, i've not tried that. i do like to give mine chews rather than treats, they last them (slightly) longer and they're good for them rather than naughty.
they do self regulate but that is presuming there's only nutritionally balanced food available, same as other animals in the wild, you very rarely see a fat one and if you do, it's most likely related to a medical problem.
if they eat food that's high (higher than hay in any case) in calories, is sweet and yummy, they'll eat that. if they eat such things on a regular basis in high quantities, it causes problems because they cut out the foods they're actually supposed to be eating a lot of - hay - to wear their teeth down. mae, the rescue girl who had gut problems, was fed exclusively on banana chips and chinchilla mix. well, that's what they were feeding her, i would imagine she was mostly eating banana chips. when i found her, she had about a week to live, she had teeth problems but the vet couldn't even do an xray because she was so severely underweight that she would not have survived anaesthetic, she has a chronic bone infection in her jaw which means that she has a tusk underneath her chin as the bone's extending as a reaction to the infection (and this cannot be treated so unless it miraculously goes away by itself, she's stuck with it), her poo was non existent and she looked awful. eventually she had her xray and dental and it was expected she'd need another one in 8-10 weeks - this was six months ago and her teeth are great. she's even eating wooden chews now! she went into stasis three times, the first time was the worst, she nearly died. this means we have to be very careful with her diet, introducing new foods etc.
i'm in no way saying that giving an occasional treat would result in this - absolutely not - she was just fed the bad stuff and no good stuff but it's a good example of what the results of a bad diet are. she's a very happy and affectionate girl