Glyfada is beautiful and your son will love the sandy beach. Keep an eye out though, as I find our children when running in and out of the water tend to drift down the beach as the waves come in at a slight angle. It is easy for them to lose sight of you unless you keep calling them back!
The beach slopes fairly gently although in places it does suddenly get a bit deeper (nothing to an adult or older child) before getting shallow again. The children love the sea here but currents can be strong. There is a flag system in operation and lifeguards patrol the beach so if you keep an eye on things you will be perfectly safe.
I haven't stayed at this Louis hotel but they usually have a very good reputation and this one an enviable location. You should have a great time!