Honestly? We do tend to run in packs... If your son has ADHD then either you or his father likely have ADHD, if you have ADHD or are attracted to people who have ADHD then, again, your friends are likely to be neurodivergent. It's not really a "magic ADHD detector" more like things like - people with ADHD tend to be a bit "alternative" and on the sidelines rather than having been part of the popular crowd at school, if you're the kind of person who is a stickler for time and sees being late as a personal affront then you're not likely to be friends with many people with ADHD (but the people who are all always late and disorganised and don't mind it in each other might well all have ADHD). If not diagnosable then certainly traits. People with ADHD traits tend to annoy non-ADHD people but not so much each other.
I'm not going to diagnose him on an internet forum because that would be silly, but I do think that you have to think a bit critically about the forms, you can't just say "Oh yes I have to remind him to do his teeth" (if you're thinking duh, every child would skip that given half a chance) - it says how often do you have to remind them to do stuff that they do all the time, ie, something you'd think they couldn't literally forget, but they do. And TBH, with the teeth issue, it's not really forgetting that's the issue most of the time, is it? Most children would skip brushing their teeth on purpose, because it's boring and it doesn't have any kind of instant effect so they don't see the point in it. They wouldn't literally forget, even if that was their excuse when you prompt them to do it. If you think he is literally forgetting (and not just saying "Oops I forgot!" because he didn't want to) then that is unusual, most children know/remember the steps in their bedtime routine.
How often does your child avoid, dislike, or are reluctant to engage in tasks that require sustained mental effort or thought?
I do dislike this one because I think it's too subjectively worded. Most people dislike tasks like that, and might even go to the extend of avoiding them if they had the option. But I think that the difference is clear if you observe the behaviour of someone with ADHD compared to the behaviour of someone without ADHD, child or adult. It's just not really made very clear on the diagnosis criteria/screener which is why it's not very well worded IMO.
With a child it will be that trying to get them to do the task that the child thinks is going to be hard or boring is like (if mild) leading the proverbial horse to water and then watching it die of thirst. If severe, it's more like trying to lead a horse to water when it's scared of water and galloping full speed in the opposite direction. Basically having to expend SO MUCH energy on trying to get them to do tasks that they don't want to do. Most parents have to remind their DC to do homework, cajole them off the console, establish a routine, offer support/bribery/threats etc but they will basically do it without you having to breathe down their neck every second, especially past secondary school age. If you're having to breathe down DC's neck and they are old enough to wonder if they have ADHD themselves and want to know (over ten?) - that's probably qualifying as avoiding/being reluctant to engage with tasks that require sustained effort.
With adults the most common thing people report is internally wrestling with themselves saying "Just go and do the washing up/write the essay, it won't take long, come on, it needs to be done, you can't sit here all night" and somehow not actually being able to activate the muscles to make yourself stand up and haul your own arse off the couch.
If you play around with the screener you can often see how many symptoms it needs to return a "yes you likely have a chance of ADHD" result - some of them are overly sensitive but some of them are quite stringent and TBH if you're ticking "often" or higher on the majority of these lists or even if you're reading down the list kind of laughing and saying "That is SO him" "Have they put cameras in my house?" then... the screener is probably doing what it's supposed to do.
I think you can seek referral through the GP. If DS wants to know then it makes sense to push for a referral IMO. The functional impairment rating scale on this site might be useful to work out whether there is impairment and if so, where it is, which would help with both a decision on whether to push and if you did want to, explanations of why/what areas it's causing problems. https://www.caddra.ca/public-information/children/forms/