If its a male I dont think he's thin. Males tend to be thinner anyway. I have some that have been "thin" for a year now. Others that are plump like a female. If you have had him 6 weks and he has eaten 4 times thats not bad. He would gain weight fast if you could get him eating once a week like Tiff said. The tank looks fine as far as clutter goes. Just try to get him eating more during the summer months so if he goes off feed during the winter it wont affect him negatively.