I don't think my meaning was clear.
Whether or not OP's son failed his degree, he's still entitled to the same level of support as alumni. Looking at their website , they don't seem to offer individual career coaching for alumni anyway. They are just following the policy and not making an exception for anybody else.
If this is wrong, and alumni have access then OP's son should too. That should be challenged.
It's not even clear whether he even emailed or just tried to book an appointment on some automated system that rejected him automatically as not a student.
However, if they have advised that they can't help because they don't know anything about certs/HE diplomas, that's also fair. You say they 'may' know. But they don't need to really. They are a university career service not the Jobcentre, FE college or anything like that. Those sorts of qualifications are not what they expect their students to be leaving with otherwise it's a failure of the university.
What he should really be asking is, instead of just an appointment or generic advice, is more targeted information.
He should tell them that he's left with a U, are there options for credit transfer or any resources regarding roles with this degree classification? Ask them what they can do.
They can then respond as appropriate.