Most puppies get the runs and it isn't necessarily anything to worry about. Changing food would be my last resort, I think.
Of course, all of what I'm about to say is no substitute for visiting a vet - it's just a process I've whittled down over a few puppies and lots of vets advice. If your pup is listless, or visibly poorly etc, disregard me entirely!
When did he get his jabs - and when was he last wormed? Often, if they're close together it causes an upset. I try to do wormer one week, jab the next, them wormer, then jab - all at a week apart. I wouldn't be panicked about a bit of blood in poo at that age - blood vessels are very delicate at that age and unsurprisingly burst with the force of the squinty ness.
Vets usually follow the same elimination process at the beginning of this sort of thing, and I no longer go to the vet with a pup until I've done the following (unless the poorly bit applies, as above).
Can you get him onto scrambled eggs with a pro biotic in it? Either some glucose powder or a couple of teaspoons of sugar in - maybe you can get hold of live yogurt or similar? Feed for a couple of meals, then start to mix in a tiny portion of really soaked kibble.
Slowly increase the proportion of kibble and see what happens. Keep the portions small though.
If there is an improvement, brilliant, keep going. If not, then I would do a concentrated worming using Panacur granules for puppies - not sure whether it's available for you though. Keep feeding a smaller portion of soaked food 4 times a day.
There may well be an improvement at this point. If not, then I would change food. There are some really good foods available in the US which the site Flatbread has linked to will help with. I'd change the food straight away too - not faff about with a slow introduction.