I would also request a Prior Written Notice stating that the consultant's recommendations (which you agreed with and asked for - if you didn't make this clear in the meeting, request another one and do so) were denied. If they refused to issue a PWN for this, your follow up would be "then you agree with them, so let's add them to the IEP." They have to do one or the other - agree to add to IEP or issue a PWN that they refused. That would be a compliance issue.
But you are correct - otherwise, you are talking FAPE and lack of transition plan, which would require due process. I would start with requesting mediation or a facilitated IEP meeting (if your state offers those).
The big problem I see is your son graduating in 1.5 months. That doesn't give you a lot of time to do anything (mediation, etc.). You may be stuck with asking for compensatory services for the school district's past failures. Are you considering delaying his graduation date at all? If so, reach back out on this site for advise on that route.
Sounds like your district is playing the "delay game" and winning.