Child dose not want to see me

My partner has a 5 year old who dose not want to spend any time alone with us any more, we have had a great relationship with him and he was sleeping over then one day he just stopped wanting to see us and crud when we first go to see him , he warms up to us and then has fun but never want to leave his mum, we have tried so many things and have no idea why.

it has happened befor when there was a lot of change on his mums side but he was happy to c come again in a few weeks this time it has gone on for months , looking for any advice