lots of small poops is a very good thing. in fact, i'd say lots of small poops is better than a few large poops. to me it shows that the gut is moving. and given that the digestive process takes 12-15 hours, i'd say it suggests that he's eating.
don't give him raisins or anything sweet, this would be counterproductive as it would create gas in the gi tract and given it's not moving as it should, you can create pockets of gas that are not moving and then you have a much bigger problem.
don't be angry with yourself, you couldn't have known. besides, from my experience, they very much appreciate having a friend once they accept one another so you really cannot blame yourself for wanting to do that.
now, don't panic. he's pooping. that's good. in fact, very good. he's having lots of little poos. that's very very good. the fact that he's not eating or moving around, imagine a child with flu. do they want to eat or do anything? no. chinchillas, regardless of what's wrong with them, stop eating and moving around if they're not feeling well.
i would ask your vet for lactulose. this is an artificial sugar liquid that helps lubricate the insides. it works very well, especially if the poops are dry, it helps it along. you also absolutely need to get food into him. i had to force feed mae on numerous occasions and it's not pleasant for either of you but it's important that his gi tract keeps moving. you could mash up pellets and put a bit of yogurt in it (he really needs the probiotics) - get a 2.5ml syringe and stick in in the side of his mouth rather than the front so you're not forcing him to swallow as if he accidentally breathes in, it could end up in his lungs. so if you just put a bit in his mouth and let him swallow it in his own time, that's super. when you're making the mixture, use warm water, they prefer it warm for some reason and when you're getting it into the syringe, make sure there are no air bubbles as you don't want to pump air into him.
as for taking him back.. i would hold off if he's still pooping as a visit can be stressful for them. they can give him gut moving medication but given that he's producing poops, it would appear his guts are moving. the reduced size and amount is probably due to the fact he's not eating as much as normal. this can take a while to get back to normal.
so, to sum up, lactulose if you can get it from the vet (it's very sweet and they love it!), force feeding and probiotics!