Excessive drinking; Insulinoma?

Hi again, if you saw my previous thread here  then you would know that my boy's weight has been fluctuating and he's been very lethargic and not wanting to play. Another thing I've noticed is that he drinks a LOT. I'm not complaining, because naturally, water is good for the body. But he'll drink for 15 minutes at a time. Could that be a sign of insulinoma? sorry for all the posts, I just really am curious as to if you guys think this is abnormal. 
Also, once again, yes I am working on trying to get my boy to the vet soon. 

I think to get him to gain some weight I'm going to feed him some soup with chicken baby food, olive oil, heavy cream and a bit of his kibble. 

It's not a sign that I've heard. 
This article is one of the better ones I've read.

http://www.veterinarypartner.com/Content.plx?A=2427


The soup is a good idea.

THE DOOKS OF HAZARD

I got some chicken baby food and heavy cream. I mixed these all together with some olive oil and he seemed to enjoy it C:

Just be aware that the cream can make for some funny poops.  I put like an ounce  tablespoon or 2 in Eddies soups for a time and it really fattened him up.

THE DOOKS OF HAZARD

But he'll drink for 15 minutes at a time.

Do you have water in a bowl, or a water bottle?

Snapper13

But he'll drink for 15 minutes at a time.

Do you have water in a bowl, or a water bottle?

I have a bowl. I heard that water bottles chip teeth, and were bad for them, etc. 

Bowls mean they can get access to water easier, too.  I was thinking that if the water was in a bottle, he may have to drink for 15 minutes to get the amount of water they want.