M.M.
I agree with the other moms, let him have his aquarium. Also keep the toys in the payroom and just books in the bedroom.
I painted my little guy's room white and used a border in the middle of the wall, about 1/3 way up, and then took one of his trucks and drove it through black paint and then drove it on the wall under the border. THen I took the same paint and painted the window sill and took white and yellow and pained street lines and a stop line. Mine had his little cars in his room. He also had one of those cityscape rugs from Lowe's, Home Depot.