I had a chin born about 3 weeks ago. He was born at 53g and is now over 110g. 350g is what I would consider a heathy 2-3 month old chin. But remember this. All chins are different. I have some full grown at 500g and some over 800g. There are plenty of chins over 1,000g. As long as he is eating and drinking and his poo looks fine then he is ok.
Pet stores don't know the ages of the chins they get in. They make guesses, usually a age a customer wants to hear. Chins vary in size. Adults can be anywhere from 450 to 1000 grams. As long as he is eating and pooping fine and not loosing weight he is fine.
My girls have wheels but I monitor their weights weekly until 1 year and monthly after that. If you give it to him and he continues to gain, since he is young, he's fine. But if he loses or stops gaining for a few weeks, I would take it away.
I wouldn't give it to him untill you have had him for 6 months. That will ensure he is 6 months old at least. he will be ok with out a wheel, but if he is to young it could cause problems for him.
I was told to wait until they are at least 600g before giving them a wheel/saucer, so that's what I'm planning to do.... so if Yoshi is over 700g I think you're good