SO doesn’t want anymore children - just found out I’m pregnant

I’ve been with my SO for over 5 years he has an 11yr old daughter and has full custody and has since she is was little. We had our own son together 18months ago. I’ve always said I would like at least two if not 3 kids and he was open to it. Since having our son his views have changed. He’s happy with his two, one boy, one girl. Where I have always said I would like 1 more. I don’t want our son growing up as an only child - due to the huge age gap like his older sister - we are dealing with only child issues and behaviour constantly with his daughter. After having our son my mental health did take a hit and the lack of sleep definitely hasn’t helped but I’m finally feeling more myself and getting more decent sleep. He thinks financially it wouldn’t be a good idea to have another child either. I got diagnosed with adhd earlier this year. After waiting to get diagnosed and being able to go on meds without being pregnant or breastfeeding. I thought the meds - I’ve been trailing a few where what messed with my period. But nope - it’s cos I’m pregnant. I track my periods with an app and have done for years and never got pregnant unintentionally so not sure how this happened. How do I tell him? I know he won’t want it and I do but I know either way - keeping it or not someone is going to get hurt. I feel so terrible and honestly feel sick at the thought of telling him. I can understand where he’s coming from - he has his daughter and son but I don’t feel the connection he does as his daughter isn’t “mine” and I met her at 6 not a little one. Any advice would be appreciated and sorry if this isn’t the right place to post, I just feel so confused