You need to rest when DS is at nursery, not be doing housework. This will take self-discipline. It is so easy to 'just do a quick tidy and put the laundry on' and before you know it you have been doing tasks for a couple of hours.
Make yourself go to bed with a book.
If necessary, put DS in nursery for a third day and ear-mark that as housework day, if you feel well enough.
Also get a cleaner to do a good weekly clean.
It is difficult with self-employed partners. To make a success of it and bring in the money, they do need to work all hours, including evenings and weekends. I would not begrudge him taking a bit of down-time during the day when he can.
Perhaps he could put two or three nights a week in his diary to be at home and put his son to bed? If it is a fixed regular 'appointment' with his son, he may find that easier to manage. Stress this is not just about helping you, he also needs to bond with his son and develop their relationship.
As he is bringing in enough money, could you cut your hours, at least while DS is so young and you are ill. Are you likely to get better in a couple of years? Or do you need a long-term solution?