Has he had wet diapers? Can he hold down liquids? If the answer is no, I think you should go to the er. They can give him some iv fluids to take him up.
Kiddies don't tolerate dehydration well.
If you don't want to go to the er, make every attempt to get fluids into him. Food does not matter at this point, only liquids. No juice - sugar irritates the intestinal lining & can worsen diarrhea. My hospital recommends pedilyte. Not Gatorade - the electrolytes are off for kiddos so young. If your kiddo won't dry k it, try getting the unflavored kind & making jello with it.
Popsicles count as fluids.
You can use a syringe or eye dropper if you have to, to get a couple of mL in at a time, but if your kiddo isn't peeing, has no tears when he cries, can't keep down any liquids, is very lethargic, or if you're Judy really concerned, it's time for the er.
Don't wait too long - better safe than sorry. We've saved many kiddies this year simply with iv fluids.