Is that the coated gravel? It looks like it. The tank looks good already and you have things in there that he'll love.
For things that could be added.. a flexible bubble wand would be great. Just get a gang valve so you can control the air flow rate, but Bettas love them.
http://www.drsfostersmith.com/Produc...4&c=3578+3671&
Our Bettas all love the seaweed decor pieces in the picture attached. Unfortunately, I've only found them at Petco.
They all have loved the rainbow caves at wally world and the T-shaped hamster tube pieces. We like to rearrange the decor in our Betta tanks each week with the cleaning and water change - it appeals to their curious nature and love of exploring.