Doesn't sound to me like you WANT to be in a triad with him and his lover.
Could just state your willingness. Like... "Thanks, for asking. But but no thanks. I have enough going on with postnatal depression and being in a "V" here with you as the hinge and your lover as my metamour. At this time I'm not willing to change to a triad model."
It also sounds to me like you would like to address things in the marriage. Which is a separate mini relationship tier inside the greater polyship. For the polyship at large to fly straight, all the mini relationships inside need to be healthy. Welcome to polymath.
What sorts of things are problems? You meantion jealousy. Could any of this help bring things to his attention in a constructive way?