Although ideally when the brain is developing you shouldn't take any mind altering substances, teenagers don't think about the long term. It's relatively harmless, but not completely harmless. It's also probably not as harmful as you think it is.
I smoke weed pretty regularly, I love the stuff, but it has caused me problems. Problems I've managed to overcome and which might not even be related to the weed. In my experience the thing weed can bring out most is anxiety or paranoia, but it usually just worsens them rather than actually causing them. I never would have smoked it as often as I do now when I was younger than 16. If he smokes it every now and then, perhaps once a month at the most, I honestly wouldn't worry about him. The best thing is to keep the honesty and trust between you and your son so you can keep an eye on what his drug use is like. Rather than trying to get him to stop smoking weed full stop, because that may not be possible and may just lead to him ignoring you and thinking weed is completely harmless so he can smoke it everyday without consequence, try and just explain that it's fine for him to smoke it every now and then but he shouldn't make a habit of it until he's a bit more mature (16-18+). Excessive weed smoking before the age of 16 has been shown to possibly lower IQ. The key word is excessive. And excessive weed smoking can have possible repercussions on the mental health of teenagers and fully grown adults.
The negative effects of weed vary a lot from person to person. I know some people who can smoke weed all day everyday and be absolutely fine, but for most people that's not the case.
Schools, teachers, parents etc. have taken the "you shouldn't smoke weed/do drugs at all" approach for decades and it doesn't work, but I do like the way you're going about this. In my opinion children under 16 shouldn't smoke weed or do drugs (including alcohol) at all. The best age bracket to smoke in my opinion would be 18-21, 21 being the best age because you've matured a lot by this age and most mental health issues which can be made worse by weed would have manifested themselves by this point.
Sorry I went a bit off topic. Blame my ADHD. I'll just briefly make sure I actually answer your questions.
What you think the ideal age is to start, if any:
18-21. The later and more mature, the better. But 18-21 is the age where smoking weed will be the most fun also.
What are the dangers at his age?
From heavy smoking? Lowered IQ, anxiety, paranoia, psychosis, worsening of latent mental illnesses which haven't quite manifested yet. Standard stuff.
From light smoking? Much much lower risk of all these things. Risk of maybe having a panic attack while stoned which could cause problems, but would most likely just put him off weed for a bit (happened with me).
Advice about effects on motivation, etc.
Most people I know who got stoned regularly at his age were lacking in motivation, but this doesn't mean they failed at life or got terrible grades. I've never seen occasional weed smoking impact somebody's motivation though. It's usually only they are high all the time. Or high when they should be studying, doing homework etc. If he is smoking it when he's a bit old, we'll say 16+, it would be fine for his motivation if he just kept it to the weekends after he'd done all his schoolwork. Getting high is a much more enjoyable experience if you do it after a week of hard work, or when you've got that satisfied feeling from finishing all the work you need to finish. My favourite time to get high is at the end of a working week (I work at a hospital so they're busy shifts). Or after a particularly long/busy/stressful day at work.
When I was still in College, it was best to abstain from weed during exam seasons and then just get high as fuck after my last exam to celebrate.
Sorry I've babbled a lot. Let us know how it goes, I'm sure your son will turn out fine but it's reasonable for you to be worried.
