Hi Margie, This inflatable would be a good option for a 40 inch tall kid to play in. There is no roof covering over the top, so even as your grandson gets taller, he can still bounce and play as long as he is within the 100 lb weight capacity. We're happy to advise in our call center, feel free to reach out to us at 877-781-8069 with any questions! Sincerely, Blast Zone Support