Firstly, I think that deep conversations with teenagers are fine, but remember that you need to be largely in control of this. You are able to steer the conversation to gently nudge him towards an awareness of the good and joyful aspects of life, whilst at the same time acknowledging his concerns. Sinking together in morbid conversation is no help to him. He needs some balance.
This sounds fairly normal for teenagers, who start to question what the future holds and what the point of life is. You can sink into his pit and drag him further down, or seek to help him over this patch in his life.
The problem here is that it sounds as though the tenor of the conversation is feeding into how you are feeling at present, which is a dangerous scenario. He needs some objective balance in the discussion to help him move on. Circular wallowing is no help at all. Letting him go round and round with conversations in the same mood will not help. Please be careful that you are not unconsciously feeding this.
It is satisfying something in you to have these discussions with him - which I presume you cannot have with anyone else. Please never forget that it is young men who lead the suicide statistics.
If he asks if you are happy, what to say? Do not under any circumstances talk of your relationship unhappiness, unless he asks specifically about this. Do not lead the conversation in that direction in your answers. Say that sometimes you are happy and sometimes less so, and that this is normal. Tell him that we can make our own happiness when things seem to be going the wrong way. Help him to see that our happiness often depends on the choices we make and help to steer him in the direction of wise choices. Bolster his confidence in himself that he can do this.
If he asks specifically about your relationship, say that it has been better in the past and that there are rough patches to be weathered. But that this is a normal part of relationships and changing needs as time passes. Do not apportion blame and leave him feeling he has to take sides. Do not give specifics - keep it honest but general. Do not feed him a level of dissatisfaction that your partner is not already aware of.
Does he talk with your partner too? Or have you two become a little enclave within the family? That is a danger too - he needs his Dad.
You have a lot of thinking to do about how you handle this scenario - mostly around being sure you do not slip into the trap of using your son for your support - that must come from somewhere else.