My 4 month pup has a swollen belly and pale gums.
He has had a hard stool this morning and had a drink. but is sleeping a lot.
I can't say for sure what's wrong with your puppy and he most definitely needs to see a veterinarian right away.
But it does sound as though he may have a bad worm infestation which is something that you will need to get treated quickly. Puppy worms are very common, and that's why all puppies should be de-wormed by a veterinarian several times during their first months.
If the worm problem isn't treated it can cause serious problems (including the bloated belly, pale gums, poor coat, lethargy etc.) which can lead to illness, even death, of the pup.
Of course, there could also be other issues causing these symptoms including other parasitic diseases, heart problems and some other dog illnesses. So, please get your puppy to your own vet just as soon as you can. Best of luck, hope he is doing much better soon.