Separation anxiety is quite normal at that age. It doesn't mean his mum is being overprotective necessarily.
It can be helpful if she says good-bye and then leaves right away - making a big production is likely to upset him, and sneaking away could break his trust in her. Once she leaves, you can spend some time playing with him to help distract him and show him that there is someone there to care for him.
Parents-in-Law. What do/did you call them?