Good question.
It would depend on the age and the probable diagnosis a bit I think. Some things get easier to cope with as you get older but a lot don't. Especially things like ASD that affect your social processing - as the school environment gets more socially complex, and particularly now there is so much group work etc, a child who was fine at 4 can struggle by 8 and be completely thrown by secondary.
I think, broadly, it's only a disability if it makes you less able. So if whatever-it-is makes him odd or eccentric but doesn't mean he needs help, then I'd hesitate to diagnose. Though you have to take a broad view of that - if a child with Aspergers' struggles socially, it can help them to understand that the problem is due to their condition, not them as a person.
Re: disclosure etc. At the moment afaik, you are only disabled if you consider yourself to be so. If an application form asks you to disclose, it's usually in the form of giving you an opportunity to do so, and if you choose not to that's up to you. However, if your employer later finds out about the condition and it either impacts your job or means you are asking for support from them, that can really tell against you. Some jobs/courses do come with a medical declaration (teacher training is one that springs to mind) and you probably would have to declare for that.
However - all of that is just the current situation. Unfortunately, you can't crystal-ball what the usual requirements will be when your LO is getting a job. So I'm afraid it will be a bit of a punt.
A last thought - branding/labelling is a common concept that seems to be taught as unchallengable gospel truth on eg Social Work courses, and so HVs and SWs will often pompously speak of "labelling" as if it was up there with making fur coats out of puppies, and they do it so much that it seems to have seeped into common English. Afaik, it comes from a sociology theory of the 1960s called "labelling theory" which broadly says that people live up (or down) to their stereotypes, so a label can both cause discrimination and limit the ambition of the individual. It's a very blinkered and one-sided view ime; and in most cases of disability any minor negative effect is completely outweighed by the benefits of therapy, understanding etc which a diagnosis will bring. So I'd take the whole "branded for life" thing with a shovel or so of salt. Or, hopefully, SALT.