If he has been alone and nothing has been disturbing him...no loud noises in your house...no loud thumping or vibrations...no other animals that could be going up to the tank...then I don't know what would be making him stressed. Sometimes I think some garters do better with stimulation...maybe he needs to be held by you...maybe he was used to that. I think that if they get used to it and then they are left alone, they get skiddish and have to be socialized all over again. I've noticed that with my easterns...they seem to get less skiddish the more I hold them and put my hands in the tank, talk to them, etc. Maybe with no contact, and the towel around the tank and all...it was so quiet for him...that when he finally had some light, noise, etc....it was a little too much for him and it startled him. I think it's better to let him adjust to things as they normally are...don't make it too quiet and secluded for him. There has to be a balance...try putting a towel over one end of his tank...the cool end. That is what I do for my snakes. It makes the cool end nice and dark and secluded for them...but the warm end is bright, and they can see out. Hold him and make contact with him, just don't do it too much. Try it for a couple of minutes each day. When was the last time he ate?