I think you should go for a hotel downtown (sort of a downtown) or find a room to rent close by. There are some apartments close to Robinson's Mall you could try. However, as a semi-handicapped person (I am out of a wheel chair but need assistance getting up and walking) I can tell you this is not a good place for persons with disabilities. It will not be that easy to navigate around pushing a wheel chair.
Going to an out of town beach will be a problem pushing wheel chair through the sand. You would probably need to hire a helper to assist your lifting of your son and help in other areas.
On a positive note, there is a nice paved walkway along the sea in town, about a kilometer + long. Many of the restaurants have guards in front who might help you off-load and going into a restaurant. Finally, there are a lot of trained nurses who may not have a job and labor is inexpensive here.
A lot of your options will depend on your financial situation. Good luck.
Note: A suggestion is to rent a place that has a shower with a hand spray unit and buy a plastic chair with arms your son can sit on for his showers.