Hi M.,
It sounds like there is more than the breastfeeding transition that is going on.
When I had my daughter and was breastfeeding I was advised to introduce her to the bottle as soon as possible if I planned to work. I tried in the first two weeks of my dtrs birth and it was a bit challanging but we took baby steps. First I would feed her once from the bottle. Once she adjusted to that I would continue one feeding a day from the bottle plus have my husband feed her once a day. I could not be in the same room if she heard my voice or me even moving around she would refuse and demand me to feed her.
I know you enjoy the bonding but, it would be best to slightly wean if he has to be in daycare. It is fine for him to go a few days w/o milk but more than that has its risks. Nutritional and dehydration. If you just started daycare that alone is a huge change for him, then to wean from the breast on top of that. It can be quite frustrating for the daycare too as they may have other children and babies to care for. I put my dtr in daycare at 12 weeks and it was hard for her even harder for me. I made sure the daycare center (Montessori) was very nurturing and loving. I paid more for this but it was worth every penny. It felt so good that since I wasn't around loving and caring for her, that she was in amazing hands. As a matter of fact, when we switched daycares, I was so attached to her favorite provider (she was the provider's favorite too) it was so hard for me. The provider loved Lexi so much and I felt I was tearing her away. We are still in touch with her, as anyone who loves and has treasured my dtr that much deserves to remain part of her life.
I know that's alot to think about. This entire transition does take time, but if you do this gradually it should work out. There is plenty of information in the internet and I am sure your sons doctor will have suggestions too.
Good Luck!