I would probably prepare yourself that he may or may not get a diagnosis at his appointment with the psychiatrist, not because he doesn't have or they don't suspect a condition but because they sometimes decide to observe for months first before deciding on one
I'd prepare to expect him to be prescribed some medication- but then be observed, it may help it may make things worse... it may need changing, it may take time to find a medication that he is not suffering from too many side effects on
I would also prepare in my experience as a previous partner to someone who was given a life changing MH diagnosis for it to shock him, no matter how prepared he thinks he is, and for it to take time to come to terms with the idea that life can still go on regardless of a diagnosis
So many people have bipolar or schizophrenia and work full time, raise families, run companies etc... but they do still hold a stigma that we are quite private and you may not know who the people are in your life who do as when managed well - life carries on
Obviously not everyone has a condition that is "managed well" - it will be up to him to work with his psychiatrist to manage his condition, I did feel like my partner became a guinea pig for medication and forgot that he could decide if he wasn't getting on with a medication and could discuss medication with a psychiatrist not just say ok two times a day to anything even if it clearly had worse side effects than benefits for him
In terms of the GP... they aren't MH specialists, they have some training... but see tons of patients for a variety of things and often signpost to specialists. It's very common in my experience for GP's to make statements that are slightly ill informed regarding mental health conditions.
It's also very common to be given a diagnosis in year 1.... and 4 years later have a completely different one because symptoms overlap in some conditions and it takes time for professionals to get to know the patient and observe them and see how they respond to treatments
With bipolar I know some can manage without medication, but do so by dramatically changing in their lifestyle to do incredible amounts of daily exercise. That's just my experience of people I know though - I doubt a psychiatrist will recommend swimming 2 miles a day but if someone comes back and says they've been swimming 2 miles a day, and been off meds for 6 months and there's evidence they've been coping well for 6 months- they are likely to say great, stick with that then, see us if anything changes.