which would you recomend for a 14 year old boy who can unicycle but is still a beginer 
A 24 is a good, all around size, used mostly for MUni, but also can be used for street, uni basketball, short commutes and track racing. The 26er is becoming the most popular MUni choice, since the tire choices far exceed that of the 24. The 26er is also close in overall weight to the 24, and still very nimble in handling and performance.
The height of the rider and inseam is usually take into consideration when it comes to selecting a uni, but many shorter and/or younger riders have had no problems riding even a 36er. In that case, sometimes the seat tube can be cut down a bit if necessary.
I agree with Muni Addict. The 26" is much more versatile. As the beginner becomes more skilled, he will probably be more appreciative of the 26" because this size wheel is suitable for touring longer distances than the 24".
Also 14, I ride a 26" MUni. One of my friends also rides a similar one, and he uses it to learn new stuff on (hopping, freemounting, etc). That is a 26" too. The 26ers are fast enough to make them ridable over longer distances, controllable enough to be able to take them along very rough tracks and versatile enough to be used for different styles (I used to use my nimbus MUni for trials). A 26" uni is fun, though I haven’t had enough experience of a 24" to give any reasonable comparison.
How tall is he now?
How long has he been riding?
What type of riding do you anticipate he’ll do most?
What type of unicyle does he have now, size?
I just switched from a 24" to a 26" after 3 months of riding, I found the 26" far better for the XC type of riding I do, it feels less twitchy as well.
My build was 26" build was 1.5kg lighter than my 24"!
I have a pretty short inside leg and find the 26" fine size wise so you should have no problem
I have detailed in some earlier posts how unicycle weight affected me during the learning process (the short answer is the weight had a huge effect when I was beginning, but less of an effect as I became more skilled). If you are looking at the Nimbus Muni 24 and 26 models, there is a big weight difference between the 24 and 26 that you can really feel, especially when one is beginning. There is also a huge weight difference between the Nimbus II 24 and and the Muni 24, so much so the the “II” felt like a sports car compared to the Muni model, and I actually had a hard time riding the Muni offroad. Now that I have been riding for more than a year, I have gone ahead and purchased a Nimbus 24 Muni that I use for offroad (love it!), and a 29 with a street/hybrid tire that I use for the road and easy dirt trails. That is the best system for me. At my present skill level, I still find the 26 (with the 3" tire) clumsy and heavy compared to the 24 with the 3" tire.
My son (14, about 5’1"), who has become a pretty skilled, prefers his 24 Muni over the other wheel sizes we have, and rides it everywhere (street, dirt, while juggling, etc). I don’t know if that will change as he gets bigger and stronger.