Open cleat

I think the last time anyone used horn cleats for jib sheets was in about 1940. It's safer to be able to release the sheets in a hurry if you're hit by a gust, so just put your hand in your pocket and get some proper modern cam cleats. It'll make the whole experience more enjoyable if your equipment works and your son will be more likely to want to carry on sailing.

Buy cam cleats.  If you can't release the jib quickly enough you could well end up capsized and your son could end up cold, wet and frightened.  There is nothing to be learned by using horn cleats; very few boats make use of them, even the cvrda use new fangled stuff these days.  Get your son familiar with things he is most likely to encounter in the near future.  The only reason why securing an open cleat is taught in the RYA syllabus is because it demonstrates how to secure a boat to a mooring.  It has no place being used for jib sheets.

Get some from ebay!

I am just starting a restoration project on a 1957 Osprey -
it is in original layout and has bronze horn cleats for
most things but not the sheets..... In fact it has no sheet
cleats at all, just a huge snubbing winch mounted on the
centre of the thwart which will probably cause the crew
some pain if they come in from the trapeze too fast! As far
as I am aware horn cleats should never be and have never
been used for sheets in a dinghy. Get some cam cleats or as
they used to just hold the sheets.

All the bosuns I've sailed have had cam cleats for the jib sheets.

Most keelboats now have self tailing winches, I haven't seen a cleat used for sheets in years.

Garry

Now a snubbing winch is a good idea, I have room for one on the centreboard casing on the forward thwart.  That should keep him busy with a sheet to pull, now I gotta find a small one which hopefully has a reversible ratchet, hopefully it clicks... these X-boxes have not won the day yet!