Can you be disabled but not have a form T2201?

I have this problem every year with TurboTax when I fill out a return for my son. The form asks if you have a severe mental or physical impairment: Yes. And yes, it will last his lifetime. Then it asks if I have a certified form T2201. If I answer no, then there is an error. Why can I not answer no? His doctor which he has seen only a couple of times in his life refuses to sign anything, and my son refuses to go. He makes no income, and has no expenses. I am not trying to claim any amount, so do I have to lie and say he does not have a disability? That is not true, and could be held against him someday. Please advise! Otherwise TurboTax is useless for him. I have asked others on the street what they do, and they use only paper returns...