advice on suitable food for a puppy?

This great site tells you which dog foods are the best:

http://www.dogfoodadvisor.com/

Get the best you can afford (and find in your area). If you switch your dog’s food, do it gradually over a week's time. Each day add a little more of the new and subtract a little of the old until it’s all new. A sudden switch can give your dog an upset tummy and diarrhea.

What are you currently feeding? If your feeding higher quality food, the problem could be that the protein level is too high for him.

You want to still feed good quality food, but the protein content doesn't need to be as high as it is in some foods out there. I prefer Acana foods as opposed to Orijen (made by the same company) for this reason. Many of Acana's foods contain oats, which are an excellent source of fiber, and is a great ingredient for dogs who are prone to stomach upset.

I would recommend trying Acana Puppy and Junior. http://www.acana.com/products/classics/puppy-junio...

If you're feeding lower end food (anything you can buy at a grocery store or Wal-Mart), I'd highly recommend looking into high quality foods.

Pumpkin (like tigrillen said) ... you could also add oatmeal, plain yogurt or cottage cheese to your dog's food to help with his gi tract until he gets better.

Any of the high-quality Limited Ingredent dry foods will be good. I don't know what you are feeding now. But something like Canidae, Natural Balance, Blue Buffalo, Innova, Nature's Variety and similar are going good. Also feed the recommended amount; and feed twice a day.

make sure you don't give him rawhide bones since those can cause diarrhea.

Otherwise pumpkin puree (100% pumpkin not pie mix) can do a lot of good for either diarrhea or constipation alike.