Oppie, Tera or whatever??

I don't like them much - and I own one - but for the very young starting out I still think they take some beating.

You might also take a look at a Topper 4.2. Has the advantage that when he gets bigger he can just move up to a 5.3 without having to change boat.

Is the Taz any good ?

I had a similar debate with myself 18months ago, and shared many of your concerns. In my case an Oppie is too slow to work on the tidal water we normally sail on, but we have a small inland lake 14 miles up the road who run the IOCA Oppie courses.
A great value Oppie came on the market so I bit the bullit and just bought it, and its honestly been a great decision.
With hindsight my then 6 nearly 7 year old daughter wouldn't have been able to sail a Tera as even with the smaller rig its still a step up from an Oppie and she's had a ball. Last year she did her IOCA level 1 over 8weeks and then sailed her boat most Friday afternoons after school, and this year she has been doing IOCA level 2 and has started competing in the clubs Junior series against kids in Oppies, Toppers, Lasers and Solos. The Oppie while not the most aesthetic of boats does provide a great platform for really learning all the basics. Most kids struggle to sail upwind and at least the Oppie doesn't immediately chuck them in the drink if they get caught in irons!

My daughter started crewing for me when she was 4 and is a damn good crew now at 8 who has a good basic understanding and I think that she is a year off of being able or big enough to really make a Tera viable, and like your son, she is tall and her slender frame hides the fact that she's just over 5st.
She's built a really strong group of Oppie friends now around the training and they are all encouraging each other to take part in races rather than being pushed by parents.

I have no desire to push her into the squad system, but for now the Oppie is really working well for her.