Constipation is kitten is not uncommon and can be if they are a little dehydrated, particularly if they are drinking less milk than before. Ensure they all have access to fresh water and know how to use it! This can mean having several different bowls (they tend to prefer ceramic bowls filled to the brim with rain water rather than tap water). Do not put the water bowls beside the food bowl or litter trays as then they are unlikely to use them.
You can also add a few tablespoons of water to their food, particularly if you do not see the kittens drink much (if any) water.
If they feed little and often, this helps stimulate the gut.
Similarly, feeding a very wet kitten food vs a dry one can help with passing stools.
Check under his tail to ensure the fur is not matted and there is not a piece of dried poo, as sometimes this can block the exit! A warm, wet piece of cotton wool can be used to gently clean the bum and may well stimulate a poo.
If you are sure he is not passing poo for several days, or if he is straining/vocalising and only producing a very small amount of hard poo, lactulose can be tried. I use it as a last resort. The issue is that this can cause diarrhoea and lead to further dehydration. For kittens, I would try about 0.1mls to 0.2mls in the mouth as a one off, and then monitor for poo. Never force feed or syringe into mouth as this can result in the lactulose going into the lungs, instead allow him to lick it from a syringe or spoon.