Natalia,
Because your lo is so young, I would suggest the warm boiled water first thing in the morning. I would not suggest any juices at this age.
If you are finiding that your lo is struggling more and more, I would then chat to your HV or GP to see what they suggest. My DS suffered terribly from constipation, and they ended up putting him on Lactulose, which helps soften their BM, and makes it easier for them to go.
Please keep us posted on how you get on.
Paula