I would definately recommend pushing for regular education. That's terrible that he would be rejected just like that. How can the school even know what specific supports may or may not be needed for such a young child who is clearly bright? Ideally, the student should have special support available, but given the choice between putting him in an unchallenging (and therefore insufficient) environment vs. a challenging one in which the outcome is yet to be determined, I would advise a challenging environment. At that age, ALL children have special needs and it's rediculous for the school to say they can't handle it. Maybe it won't end up being a good fit, but there's no real way to know unless it is tried. Your son could end up surprising everyone and teaching his teachers a little bit about diversity, tolerance, and respect while he's at it .
Unfortunately, I don't know anything about the education laws in Spain, but maybe you could seek out a misdiagnosis if necessary to get him the education he needs and deserves. Or maybe private school or possibly even a gifted program, if available and applicable. I doesn't sound like a special education school would be a good fit at all. Boredom can be one of the worst things for a child to deal with at school.
Ugh, what a horrible a predicament. Don't give up and good luck!