I always pray very intensely whenever I answer a question like this, because I know someone may be reading this who is contemplating suicide - and I don't want to say anything that might encourage them to take that tragic final step. God loves them, and He wants to surround them with His love and give them hope. And He will, as they open their lives to Jesus Christ.
God knew your son's mental situation, and suicide (if that's even what it was in your son's case) is not by itself the unforgivable sin. The only sin God cannot forgive is the sin of continually rejecting Him and scorning His offer of salvation. The Bible is clear: "He who has the Son has life; he who does not have the Son of God does not have life" (1 John 5:12). If your son had committed his life to Jesus Christ, you can be confident that he is now in God's presence forever.
May God use this painful event to draw you closer to Himself. Tragedy makes some people bitter and causes them to turn away from God; it may also fill them with undeserved guilt. Don't let this happen to you. Instead, turn to Christ and ask Him to help you discover His strength and peace during this difficult time. His promise is for you: "In this world you will have trouble. But take heart! I have overcome the world" (John 16:33).