There are certainly good reasons for both you and your DH's arguments, but his are not coming across. I wouldn't even address the practicalities until he has explained his reasoning.
Who would benefit from you working ft?
What would be the benefits?
What drawbacks are there?
What are his worries with the current situation?
Just keep asking him, tell him you can't see where he is coming from. If it is just about money, again try to find out is it for immediate benefits (holiday, move house?) or for general future financial security.
On the surface, you have worked very pt for many years, and if it's low paid as well, the financial contribution may be very unequal. Of course you contribute in other ways, but you have been on a low income for a long time. However, 3 dc, one of whom is a toddler and also SEN to consider - it seems many mothers would prefer this (I would) in this set up.
I agree it doesn't matter what others do. But if he starts comparing, he must know in the SEN community, it's pretty rare to have 2 ft working parents.
Good luck op, it needs to be addressed so resentment doesn't build up. Once you understand his reasons better you can consider his position and take it from there.