If your using formula I’d recommend speaking to your health visitor first but maybe changing the milk as I had the same problem with mine. Also the pre made formula is thinner and helped when he was on the old brand we used x
How long has it been since he poo’d? Newborns can go a good few days between
If you are a breastfeeding mum, try and eat kiwi,take some oranges and drink plenty of water It will make him poo when he breastfed