Chickenpox vaccine...how long to wait after possible exposure??

[Emsillyh]

Hello all, first post here after using this site for my late night answers for many years!! I decided that I would vaccinate my son against chickenpox, if he'd not had it by age 6. My son has autism (with high sensory needs, touch sense is super heightened). His friend at school came out with chickenpox and we were told by the teacher, that was 19 days ago. In between then and now, he caught threadworms (yuck!!) which was basically torture for him with the itching, it was soul destroying. So I've decided to vaccinate asap. BUT there's not much point if he's going to come out with it. How long should I wait? I keep reading 14 days maximum, then read some posts saying 21 days. Could I therefore book him in for day 22?!
Second question, for those of you with ASD children please pass on your experiences in case he does get it? I am so worried for him :( he's 5 now by the way. Thank you oh wise mummies!