I agree with Nossie in that you do NOT want to squeeze your poor fish! You might do more harm than good!
Adding the aquariun salt to the tank might give him some electrolytes. Drop pieces of epsom salt in front of him and see if you can't get him to eat one. Epsom salts are a great laxative.
After he is over his constipation you may want to change his diet to live/frozen food, worms live, frozen or freeze dried and chopped spinach, red lettuce, peas and oranges.
He is an older fish and older pets need more fiber in their diet!
Good luck and keep us posted!