We had the same issue, DD2 has been difficult from conception with me in and out of hosi all pregnancy and then her behavioural issues it wasnt until she was 3 that I even mentioned another baby, we had a chat and decided that if she would be about 4 when the baby was born it so turned out she was 4yrs 7 months when he came along.

Why not negotiate on age gaps, it seems your 2nd baby is quite young maybe he just needs some more time before he thinks about a 3rd baby