An old fashioned remedy is a little dob of carnation condensed milk mixed with water for him to drink, you can now buy tubes of carnation in tescoes which is better than a huge tin.
Because it is sweet they usually get round to drinking it eventually lol, mix with warm boiled water.
Give until they have action and by then they will shout for it if they dont get thier fix LOL, ween em off it.
To stop the constipation try mixing their food with water, its made such a difference for my 18yr old Ducha.