You're such in a difficult position and I understand how frustrating it is to not be taken seriously. In this situation, you can either leave things be and expect the possibility of your son getting worst regarding his depressive behavior. Or go with what you believe in, despite the protests of the people around you but making sure your son gets treated for his own good.
But before doing this, make sure to win his trust all over again. It might take time but at least you have done something to improve the situation. Some teens may appear okay to some parents but if we look closer, sometimes we get deceived by what our teens say. There are a lot of reasons why most of our teens today is experiencing depression. Take a look at this example
If you think your son trusts his mate's mum more then do an effort to know what he likes and bond with him more often. That might just be one way to win his trust and eventually be able to encourage him to do treatment or counselling willingly.