If he is 10 years old, you should really consider talking to him--not once, but several times and trying to understand what he wants and is willing to do. Especially since you're willing to NOT move for your childrens sake.
Are you trying to preserve the German that he knows?
I suggest you pull together as a family and try to bolster his 3 languages with books, playing games together and conversing in all 3 languages, perhaps the family can rotate languages every 10 days or something if you are worried that he will not develop a mature level of fluency in any language. That way each language gets used in a daily setting every month. Be sure to discuss things like bills, meals, games, morals, responsibility and general hows and whys of life in Romanian, English and if you want to preserve it German.
Read books aloud and as a family in each language on a regular basis and talk about the story that you've read in that language.
What is his predominant language?
Is he comfortable in his ability to learn Spanish and cope in a Spanish culture?
Personally, from what I read in your post, I'd go to Spain if there was a good opportunity there for the whole family, you know, for the greater good as opposed to JUST your 10 year old. I dont know how many children you have though, maybe you could encourage the children to speak only English to each other within the home, Romanian with you and your spouse and German outside the home.
You could buy a few good Spanish language books now and begin working on it together and reading about the culture and education system in Spain etc.
This isn't a decision a stranger on a message board can make for your son. He is 10 years old, more than old enough to begin thinking for himself.