special perscription food - how long?

My Howard has been on special, expensive prescription food W/D since November. is this forever? how long should he eat it? he has lost weight , he is active and takes insulin very well.

but I do think it is making him constipated. It is wet food and i water it down. This just started abouter 4 weeks ago. i think his body is done breaking it down.

have any of you used the special wet food and for how long?