His symptoms do not seem to be pointing to any one problem. The lethargy could have been partially caused by his constipation but once he was able to get his bowels moving again he should have perked up. Give him a thorough once over and see if you can feel any abcesses, these can cause some of the symptoms you have mentioned and will cause him to be docile until it bursts. Try giving him more of a variety of fruits and veggies, limiting the lettuce as it is not the best nutritionally for hams. Give him fruits that have high water content such as apples to help keep him hydrated since he is not drinking much water. Also, try giving him his normal food but if you see he favors particular bits of it then give him extra of those. Also, at your local pet store they sell vitamin enriched treats for hams and vitamin drops give him plenty of those as well.
Unfortunately, with hamsters it is hard to diagnose illness in them because they do not show pain and are so small and without a vet to check him out the best you can do is keep him comfortable and monitor him. Also, try checking with a pet store that sells hamsters and see if they have a vet for them. It may be that you have to travel a little out of your area to find a small animal vet. J
ust give him lots of love and attention. Your hamster is about middle aged for the normal life expectancy for a hamster but there is no way to determine how long each individual ham will live.
Hope this helps!
Hope this helps and if I've answered your question don't forget to please accept my answer and to make it twice as nice a bonus (no matter the size) is always appreciated!
Thanks,
Rebekah