I don't disagree with most all of the advice you have recieved.
I can't imagine putting up with what you want for your life (in a partner) nor can I imagine putting up with what he's saying he wants (in a partner).
I could be friends-but no way could I date a person with either expectation.
The thing is-we all have hard limits (things we can't compromise on and still be healthy and happy) and soft limits (things we can compromise on and still be healthy and happy). They aren't the same.
We have rules or boundaries (I refuse to choose a term because there are too many arguments over which is what) as a family (all however many there are now of us), as a household (all 5 of us), as parents (all three of us), as couples (each group of 2 of us) and as individuals.
We communicate, discuss and alter those to best meet the needs of whichever group indicated.
Something that is an agreement/rule/boundary/expectation for our household is that all three adults will ensure to work their "date out" times (regardless of it being romantic date, friend time, work time or any other OUT time) with each other so that there is always an adult available to the children.
Another is that we don't any of us have sex with an additional party before we let each other know.
Another is that we don't start romantic overtures (defined by us as couples) with outside parties prior to letting each other know.
These are things that work for US.
But they may or may not work for someone else.
I think it's brutally important to back up from considering what your partner wants until you establish for yourself which of what you want is a hard limit/soft limit for you to be HEALTHY AND HAPPY.
Because throwing yourself under the bus to ensure he's happy "because you are his only family" is actually abusive to yourself and him.
You each need to establish for yourself what your hard limit needs are-then you can address if it's feasible for you to each be healthy and happy TOGETHER. If not-there isn't really an option. Because choosing to be unhealthy is self abuse.
"Love As Thou Wilt"