Oh dear what a sorry, sad situation to be in- for both of you really. What a shame property and financial matters were not discussed and formalised before you both got married.
You say he has told you he is leaving in 2 months time- the same notice period he would need to give his tenant. Has he said he's going back to his property and given the tenant notice- have you asked?
Letting him leave will provide you both with the opportunity to take stock. Both of you can decide whether the break is permanent or temporary whilst resolving your differences. Letting him go would stop the leaving threat cycle also. You could suggest going to relationship counselling together and consult a solicitor at the same time to understand your financial position and options. Counselling will provide the opportunity to discuss your feelings for each other, like do you both still love each other and want to work through problems or part? If you decide to stay you can work through, with the counsellor, how you can both manage to adapt to your differing wealth levels. You both can protect respective properties within a legal agreement as a result if you decide to stay together. You have the benefit of inherited wealth, rent from one property and a mortgage free home you currently live in. He has a state pension, £500 shortfall on a mortgage and currently garden centre employment income but that may not last for many more years and his financial status will be even more precarious. Realistically he has and will continue to have far less available spending income than yourself- do you love him enough to adapt to that if you stay together?
You are not being unreasonable being upset by your position. Protect your wealth by all means, in the right way, with legal advice. Be respectful to the man you married by going to counselling (together if he is willing, alone if not) so you can part amicably and fairly or stay together amicably and fairly.
Good luck.