It can be either.

A training certificate is aimed at juniors to the sport, so anyone between the ages of 14 to 16. However anyone of any age can get one.

The limitations of a training certificate are that the person getting it must always be accompanied by the "original" license holder and they must supervise the training certificate holder. The person with the training certificate cannot be in possession of the firearm by themself nor can they store it in their home (if they lived separately to the "original" license holder.

As your Son is 16 or older he can get a joint license on the firearm. There are no limitations and the license, while referred to as a joint license, has no such legal standing, meaning it is a full license in every way. This allows your son to possess and use the firearm whether you are present or supervising him or not

If applying for the full license, in section 4.1 of the FCA1 just include your license certificate number and it'll be known he is licensing your firearm

I'd err on the side of a "full" cert, since there is no disadvantage or needing to re-licence if within the 3 year lifespan he wants to go out shooting without yourself.

Otherwise it would mean applying for a full cert at that time and paying €80 on top of the €40 a training cert costs.
A training cert also does not get a renewal like full certs, ie you will not get a prepopulated form out for a training cert like a full one.
Bar the €80 fee there really isn't much downside to a full cert at 16, and a fair few upsides.

And yes, your son will need to prove competence, whether that be by the NARGC course, CSI, IFA, gun dealer's proficiency course, etc. Your clay club may even do a competence course, it'd be worth inquiring.

IFA and CSI do discounts on their courses for members also, so that'd be worth looking at to cut the cost if you are a member of either.

Thing is courses have been around for a long time now, so what happened before them isn't really here or there, times have moved on and likely not for the better.
Back in the day near everyone learned from their fathers, but I haven't heard of any Super accepting that for a long time.

I don't have much issue with the cost(which is not too high thanks to the competition between orgs for competence courses) if the course content actually merits the cost, but not as a tickbox exercise.

Someone being competent to shoot and competent to train someone else also isn't necessarily the same thing, hence why you don't have John Joe from down the road doing competency courses. Everyone who has been on any range(civilian or military) for long enough has seen some carry on which would bring into question the competence of those involved.

There is also the question of what proof can be provided by an ordinary licence holder that would be acceptable both the AGS and the law, "proof of competence in the use of the firearm concerned,". Which also raises the interesting point that if you do an NARGC course that for example that covers shotguns and rifles, would you then not be deemed competent to licence a pistol?
Something that I think was twigged by course providers since the last IFA one I saw organised noted shotgun, rifle and pistol specifically.

A lot of guys out there who are normal licence holders would be competent to teach someone how to shoot, the point is that I doubt it would be acceptable to AGS, particularly given how the law is worded.

I was asked for a course certificate when I was applying for my young lad to be licensed on my shotgun. It was by the FO. I asked him what was the point of the training license if someone has to be shown to be competent to attain one in the same manner as a person seeking a full license would.

We went back and forth with him not really understanding either my point or the whole idea of a training licence, which is to gain competence before getting a full license, and so told him to just submit the application as it was.

The training license was granted, especially as I had included additional information with the FCA1 stating that the purpose of the training license was to obtain experience and competence under the supervision of someone deemed already to hold those values, me.

Not saying it's guaranteed but I presume the Super had enough common sense to understand that to demand the same level of competency from someone applying for a training license (the clue is in the name) as someone applying for a full license was contradictory and perhaps somewhat asinine.

To be fair it was his first time holding a gun so some allowances have to be made 😂