How To Treat Constipation Of A New Born?

my grandson is bottle with formula and breastfeed and is now having huge problems with constipation and constantly screams what can be done I have suggested the massage with leg movement gently up and down rest try again - would boiled water or prune juice help - they are giving him gaviscon but don t know whether a doctor has advised this - I would have thought gripe water at 7 weeks would be enough with boiled water in between