Hey feeling Sad

I'm not sure if I understand the situation but it seems you feel you let your friend down by not responding appropriately. If I were in this situation I would say exactly that to him. I would just say:

"When you were talking to me the other day, after I thought about it I felt you needed support and I wasn't there for you. I just want you to know I'm here for you if you need me."

All of us fall short sometimes and people are usually forgiving if we let them know we genuinely care by saying, "hey, I goofed."

Hope this helps!