It sounds like the vet might have treated him with Invermectin anyway if he put some drops behind his ears - just in case it was mites
With the heat, it could well be a bit of a moult as well and the daily linseeds will really help his fur and skin. You could always put the drops in a mushy treat like a bit of banana or something and see if he will eat it. If he tastes the vitamin drops he might not eat it.
Ok so Chipsi is one of the better wood beddings, but I would really try to get him on a hypoallergenic bedding because he could well have an allergy. Can you get any paper based bedding in India? Kaytee Clean and Cozy is a popular one. Also have a look at Rat Rations online if you can't find anything. They used to ship just about anywhere in the world. Although if the postage is too expensive that's not really an option.
They sell paper bedding. You'd probably need to put it in the cart and go part way through checkout to find out the delivery cost to India.