First of all, I am so sorry for your loss. May your son rest in peace. To answer your questions:
1. Is suicide parents' fault? Sometimes it is. I know that suicide is a personal choice, but parents could
drive their kids to commit suicide. However, kids with loving parents also kill themselves, so it's not always the parents' fault.
2. Is your son's suicide your fault? I don't know, and no one on here can give you a true answer. I think it would be best if you saw a therapist to help you grieve your son's death. Talking in depth with a professional could give you clarity on the situation and hopefully a clear and honest answer to your question. You mentioned you still have 2 kids; if you feel like you were a bad mother to your 14-year old son, going to therapy could help you become a better mom for your living kids that still need you - especially while they're also grieving their brother's death.
This might sound harsh, but you won't get any honest answers here. We don't know your relationship with your son; some members will try to make you feel better by saying it's not your fault while others will say it is to project their grudges against their parents onto you. I really believe talking to someone in real life (even if it's not a therapist) that understood your relationship with your son would be more helpful. I am sorry again for your loss, and I hope you heal from this tragedy.
Thank you for your advice.