The restaurant at the El Paso is self service.
It has a HUGE range of food, hot and cold, served in all sorts of ways. Different food most nights. Anyone could find something they lke there.
There are loads of salads and other coldfood, main course hot food (chicken, beef, lamb, steaks, stews etc), chips and other potatoes, rice and pasta, loads of fruit, cold puddings (jelly and ice cream for the kids)
There is a picture of the restaurant here
http://www.portaventura.es/media/img/HotelesHo telEl%20PasoRestauranteMerida/HEP40102e.jpg
In the foreground you can see the self service for the main courses (that is only about a third of the length of the self service area)
Behind, with the red tablecloth, is where they put the bread and rolls (nice varied selection)
In the far distance (with the two customers with the white tops) is the area for fruit, puddings, ice cream etc)
The three Port Aventura hotels all have other restaurants separte from the "main" restaurants. You have to pay for these. If you have a half board deal you can eat there for a discount (in any of the hotels, not just the El Paso).
You will have no problem finding something for your son to eat.