How big is the bowl? Do you have a thermometer? If he's in the sun, it may cause the temperature to fluctuate too much. You should probably be cleaning his bowl every week at least, depending on the size, because it doesn't sound like it's filtered.
All the signs you have mentioned do not sound good. My bet is on fluctuating temperatures and possibly overall temperatures that are too low. If you can, move him into a 2.5 gallon tank and add a heater and a filter, then move him away from the sun, which can promote algae among other problems.
