Keep this in perspective.
Spousal maintenance is an extremely difficult order to have made. It's generally only reserved for those who were married to high-income earners, and usually only for a short period while property settlement is being determined. Unless you're in the vicinity of a six-figure salary - which I'm guessing is unlikely if you're working two jobs and share no assets with him - he probably won't get it because the court will not severely disadvantage one party to support the other.
I would ignore the solicitor's letter. The solicitor has likely advised his client that spousal maintenance is probably a no-go but that he can contact you intimidating-lawyer-style and 'just see what she says'. A letter from a lawyer does not legally compel you to do anything, so I would ignore and let your ex decide how much of his nose he is willing to cut off to spite his face.
Basically, don't enter the conversation until it actually becomes a conversation. Spousal maintenance is practically the more challenging kin of property settlement, which doesn't need mediation, so it's not a conversation until it's in the court.