I don't think it is about being forceful, I think it is about being clear and specific. At the moment it is awkward, argumentative and miserable for all of you!
Do not assume that he will pick up what is the obvious to you. State clearly what you want and why and also take into account his needs so as I suggested upthread have times when you cha to him, probably separate from when you call to talk to Mum. You can then say clearly that you are ringing for a chat with him OR you are ringing for a chat with Mum.
These sort of situations are best dealt with by honest discussion rather than hints, aggression, passive aggressive comments or whatever
Good luck 