Losing a cagemate definitely can cause issues with a chin losing weight! It puts a lot of stress on a chinchilla when a friend dies, that releases stress hormones and causes upheaval in the chin's body leading to the chin not feeling well. Weight loss and a depressed mood definitely are common in chins that lose cagemates.
Right now your chin is eating, so that's a good sign. You need to give him more attention and love for awhile. Some older chins do pretty well with getting a new friend since they are so used to having companionship. I often pair up chins that have lost cagemates so they feel better.
Recently I had a little guy lose his friend. He didn't look right maybe three weeks after his buddy died. As soon as I put in another little old male with him, he started gaining weight and it appeared he was in a much happier mood. I don't know if that would work for your chin since he was used to being around a female...but you could try getting him a male companion. He's older so it may be easier to introduce him to an older male.