Advice needed please

I just had to switch my cat over to a low fat food. I put down a sample of different ones to see which he might take to. He has always been a finicky eater. He took to the Science Diet W/D. Now he has decided that isn't for him either. He hasn't eaten much for two days. We have supplemented some cooked egg white and chicken breast meat, but he really only nibbles a bite on that.
How long do we force this issue.
He must eat the new food, and until he eats enough we can't give him his Rutin(a supplement needed for his condition, chylothorax) because it makes him nauseous.
Thanks for any advice on making a cat eat a food it doesn't want.

If it's a MUST that he eats a weight reduction formula by prescription, I would talk to the vet about one of the Ralston Purina foods. From what I have heard by talking to people, some of thier prescription diets are more palatable. You can also ask them if you can get it in a canned formula, sometimes it helps as it's a little change.

I know with some of those need specific diets, some people will add a little bit of the pureed gerber meat (for babies) and mix it into their food (but just a little.)
Things like the turkey or veal! It can make it very appitizing, and the animal will gobble it all up!

Are you allowed to add water to this food and mix it up, perhaps even heat it up just a little? Sometimes that will be enough to cause the cat to eat it.

As far as actual foods go. I can't offer much advice. I've never had an animal on a 'special' diet myself, but some former clients have.
Have you spoke with your vet and considered a RAW/BARF diet for him? That way you can control how much protiens, minerals, fat and moisture your cat gets. Normally if you have a picky eater, animals will thrive on this diet. When you feed kibble you can be a bit limited that way.
Often on the Low Fat, or Weight Management diets, they take away a lot of the protein source (aka they take away a lot of the meat.) So in the end your left with a diet that is mainly something like CORN!

We have been watching the protein levels to make sure they stay up. The specialist called this morning and told us that getting him to eat for now is more important. So we have put his old food back down and will try to slowly introduce the new.
I tried the chicken baby food and he tried to bury it
We are also giving him cooked egg whites and cooked chx breast meat. He has nibbled on that.

Thanks to all for the imput.