I agree, it's going to take longer under the T5 10.0 UVB tube and a lot more live insect protein every single day for him to start growing, he has some catching up to do from being under that compact
UVB light for the first 3 months, but the good news is that they continue to grow up through their first year and continue until they are around 2 years old. That T5HO 10.0 UVB tube is one of the best he could be under, so his appetite and his energy levels should start to pick up dramatically very soon, but it does take time.
If he's around 4 or so months old, then as AHBD already mentioned he needs 2-3 live insect feeding sessions every single day, each one lasting 10-15 minutes, where he's allowed to eat as many live insects as he wants to. 12 crickets a day is a fraction of what he will want to be eating soon now that he's getting good UVB and UVA light. I suggest that you start ordering your crickets, roaches, or BSFL/Phoenix Worms online in bulk, from a place like
www.dubiaroaches.com (they are cheap and have all of the staple insects that I listed), as you need to figure that he's going to need to be eating around 40 or more live insects (smaller than the space between his eyes) every single day. So you'll save a fortune on crickets, most people with dragons don't buy their live insects in pet shops as it's extremely expensive. He's going to need to be fed this amount of live insects until he's around a year old in order for him to grow properly.
I use Phoenix Worms/BSFL as my live staple insect, size large you can feed to even a baby because they are soft-bodied and they also provide a good source of
hydration. I order 1,000 size large per month from
www.dubiaroaches.com for about $23, and that is now feeding one 7 month old and one year old who is now eating more greens/veggies than insects. Figure at least 500 for one juvenile dragon every month, same with small or medium crickets. And if you gut-load your insects with the same greens and veggies you feed your dragon that will also help with
hydration.
It is just going to take some time for him to catch up. 5 days under a new UVB tube isn't quite enough to make a huge difference...just in case though, you said you have the Reptisun 10.0 T5HO UVB tube, correct? And it's what length, 22"? Is it at least within 10" of his basking spot? It's okay for the 10.0 T5HO tube to sit on top of the mesh as long as it's within 10" or so of his basking spot/platform...Also, make sure you have both the new UVB tube and his bright-white colored Basking Bulb right alongside each other, both over the Hot Side of the tank, sharing the top of the Hot Side of the tank, with his basking spot/platform directly underneath both lights. And both lights should be on for at least 13-14 hours a day...