As oceanauk says, none of us are experts, but you don�t need to be an expert to realise what the problem is here. It is obvious that the root cause of this problem is the fact that the child is spending half of each week living in one location and half in another. Try doing that yourself and see how you get on.
Instead of sitting down to discuss how the symptoms may be addressed you need to sit down and decide how the cause is going to be managed. It is completely unnatural for any child, let alone one so young, to live in this way. If the situation persists the behaviour that the child is demonstrating now will be but a drop in the ocean compared to that which he will develop as he gets older.
Children develop best when they have a settled life with regular routines. This child is being torn between two locations and two lots of people, week in, week out. He is unable to speak to express his frustration so it is not surprising that he shows some signs of distress. Worrying over whether he has six or eight bottles each day will not alleviate that. His parents need to put aside their own problems or preferences and arrange for their child to enjoy a more settled lifestyle.