Sorry you're in this situation. I'm not a seasoned veteran so hopefully others will chime in but the general consensus here is avoid relationship talks and pressure. If he broaches the subject on his own mostly just listen and validate his feelings and try not to get emotional and argue (easier said then done).
How old are you, your husband, and your child?
If he's already engaged a divorce lawyer, I'd consult one of your own ASAP, if only to know your rights and how things will likely play out IF it goes down that path. You don't go into detail regarding the special needs , but assume that may be a factor in custody.
Are you doing anything for yourself? Working out, new wardrobe, activities/hobbies, meeting people?
Me:39 Ex-W:37 M:7 T: 9 S:6 D:3 BD/IHS/Confirm EA/PA: Feb '20 OM1 affair ends: May '20 W/OM2 & moves out: June-July '20 W files for D: Jul20 OM2 confirmed: 9/2020 Divorced: May '21