My woohoo - DS has finally stopped calling me Daddy

My Boohoo - He now calls me "Boobie".

So my question - he seems to struggle with the letters M & N. He seems to replace them with a B. Hence the Boobie for mummy. He calls a nappy a "bappy". Cows say "ooooo". He won't say milk at all but says water quite clearly. He has quite a variety of other words and sounds he says regularly and clearly (dog, cat, don't, stop, daddy, poppy, door, car, apple, water, roar etc). Does anyone else's 22mo have this problem? Is it something I should worry about? Or is he just too busy mastering the other sounds to bother with M & N?