It sounds to me that he is crying out for attention, Is he used to having his mum all to himself? Sounds to me that he is getting the wrong kind of attention.
The best advice would be not to rise to his behaviour, set out some ground rules, like set bedtime routine etc. Then set a time in the day when he can have sometime on his own with his mum alone, then time with you altogether.
Hope this helps you, it will get better with time though.