Has your mother's estate been disposed of?
If it has, and it is more than 6 months since the grant of probate was made, then the will cannot be contested. His opinion is totally irrelevant and I wouldn't worry about it!
However, if she died recently and the estate is not finalised that is a different matter. If this is the whole story then, based on all the facts above, I wouldn't have though he has much of a case. I can't see a doctor standing up in court and saying "I don't have any medical evidence of this woman's mental capacity, but I can show the court evidence from lay persons who stand to gain if the will is set aside!"
Also, bear in mind that, if this ploy were to succeed, what would happen is that the will would be set aside and intestacy rules would apply (unless there still exists a previous will made when she was of sound mind!). If your mother was a widow, then next in line would be her children (equal shares)! Do you have any siblings? Are they the other interested parties that you mention?