I think what Opus is trying to say is that the bowl Nigel is in lacks a lot of things a healthy turtle needs. He may have grown so slowly because his habitat and diet need upgrades. Here's a link on a basic setup for a RES:
http://www.redearslider.com/startup.html
None of the things on the equipment list are options - they are all essentials. If you need help finding some of these things in Dubai let us know - we can probably figure out equivalent work arounds.
On the diet - I suppose you are now trying to wean Nigel off the steady diet of shrimp? Keep cutting back the shrimp and increasing the pellets. Eventually, he'll be able to have some shrimp every few weeks but it won't be a daily thing.
on the quantify of pellets, the reason the directions aren't more exact is that it depends on the size of the pellets as well as the size of the turtle. Some pellets are a lot bigger than others. What you should do is imagine your turtle's head - not the neck, just the head - and then imagine that it's hollow, like a thimble. Then estimate how many pellets it would take to fill that hollow head. That's how many pellets your turtle should get every other day.
There was a really nice thread here a little bit ago with pictures of how to estimate the number of pellets. I'll see if I can find it.
Then, some pellets are more nuitritious than others. Some very good quality brands are reptomin and mazuri. There are some other good brands. What brand of pellet are you feeding to Nigel?
That's very good news about the lettuce - I mean that he's eating it pretty well. He may not eat it every day but you should offer it every day - or you can offer other types of vegetables like a little piece of a carrot or a tomato or some kale sometimes. Blueberries are good. For the lettuce, make sure it's green or red leaf lettuce type and not plain iceberg (or white) lettuce as the iceberg is particularly non-nuitritious. I don't know exactly what types of veggies are available to you in Dubai.