6 week old constipated

My 6 week old son has been really constipated so I called my health visitor and she said to try giving have an ounce of cool boiled water in the morning and again at night I’ve been doing this for 3 days nearly and it still hasn’t helped him I have called her again but there was no answer, is there anything else I can try and do for him I’ve tried massaging his tummy and moving his legs in a cycling motion but still nothing’s helping him, he is combined feed I think they call it breastfed and formula but my health visitor said it wont be anything to do with that, I just want to help him he strains so much to try and poop I feel so bad for him.