Should I try for one more?

My son is 2 1/2 and he's just been diagnosed with autism. I'm so up in the air. I'm crazy about my little man and all his quirks but I'm not delusional about all the things he will face in life. I've always wanted 3 children! I now have two beautiful boys. I'm 40 years old. The clock is ticking! My oldest is 12, so my view is, my youngest would have someone close in age, a sibling, to be his best friend. What are your opinions. Please keep it thoughtful. I'm already struggling with it enough!