It may not be as bad as all that. My parents have split up and both remarried but they still remain in regular contact.. and sometimes share relationship advice.
I think the thing is here that he is gently having another relationship but you are quite quickly seeking an all-in one. Nice to want it but perhaps a bit premature to expect it.
Them sharing a room could be faux married behaviour or could be a close friendship. I say this because I have been like this with exes. .but no not with someone I have been married to.
It really depends on how they are with each other but I understand that you can't judge this.
It may be that their currunt relationship is slightly underpinned by a frisson of possibility that your involvement in would take away.. and he could be being sensitive to this and/or any craziness from his family.. Theses things can get quite sticky once it is unavoidable that big changes have taken place.
I agree with others that it is early for this to happen - a few months in would be very short for me in an adult relationship with other responsibilities around it. I appreciate this came from friendship so I can see where this comes from but you may have to accept that your pace of development isn't his.. at least for the moment.
A chat along these lines might help and be prepared to listen. Personally if I were in this situation I would want the current relationship to be more open in this regard. . he msy also be wary of getting into another big investment relationship so it might be wise in any case to not push on the heavy stuff as it might push him to think that any relationship is a hassle and 'perhaps I need time alone'. Again from my perspective I would be seeking something simple friendly and serious withput being pressured ahead of what I thouggt were reasonable timescales given all the people involved.
My advice would be to try to enjoy now rather than future and seek a discussion about how ypu wpuld like to move on to something more serious if there is a future. Though I keep in touch with some exes I wouldn't introduce current partners to them as a deliberate project to prove my love for them, but I knowsome people do. On that its horses for courses and you shouldn't necessarily expext that what you would do is right and that what they would do is wrong.
Good luck and keep being aware of how you feel and how to explain it.