I used to live in ibiza and found that at aged nine, there were a few i couldnt go on, but i was short for my age (still am!) im sure they will let your son off if there are just a few inches in it..
I was able to go on about 99% of them.. there were just one or two that i couldnt though i wasnt a good swimmer, so if you explain to them you allow him on as he is a good swimmer, i dont see why they shouldnt let him on.

There are two waterparks on Ibiza.

Aguamar in Playa D'en Bossa and Aqualandia at Talamanca.

Was it any one in particular you were thinking of ?

John.

I believe the largest one is outside of Playa D'en Bossa

Hi Ash,we went to the waterpark in Playa den Bossa early June this year,the youngest in our party was 11,so was able to go on all the rides,if you ask the rep at your hotel they should be able to tell you,where are you staying,it may be possible to check it out for yourself.Any questions,I'm willing to help.

Hi Ash,please see link below,this may help you;

Click here

think aqualandia is closed and has been for ages......

hi
we went to the water park last year my son couldn't get on some of the slide's. We went back when someone different was in charge and he got on so just depends on whether you get a jobsworth or not. However there were plenty that he could get on if your lad is tall for his age he will prob be ok

You may be able to find information about the height restrictions on the https://heightscomparison.com/ website or by contacting their customer service team directly. It's a good idea to ask for details about the specific attractions your son is interested in, as well as any general height restrictions for the park.
It's great to hear that your son is a proficient swimmer, but keep in mind that height restrictions are put in place for safety reasons and it's important to abide by them to ensure everyone's enjoyment and safety at the park.