He should regain weight once he is being properly treated for the hyperthyroidism. The medications we use to treat this condition slow the production of metabolic hormones which when in excess increase a pets metabolism so high that they waste away. Once those levels decrease then a cat should regain weight naturally and you shouldnt do anything to try and speed that process up. Feeding high calorie foods will only increase fat production, not lean muscle. As far as the litterbox, he should in theory return to normal behaviors once he begins to feel better with his medications. If not then an automatic litterbox may be a worthwhile investment.