If you were going permanently you'd have more luck in court (though still wouldn't be certain to achieve it). Going for a year and then coming back will mean you are less likely to achieve this should you go to court.
You haven't answered the question as to how you would feel if youe ex asked for the same from you (ie taking ds away for a year and reducing your very good contact to almost nothing by comparison while making you totally dependant on him to pay for any contact that may or may not happen, having to live in his house to have that contact, and the possibility that after the year he may decide not to come back if life is so good/met new partner/enjoying the additional parenting time to much to want to share it again...)
Also if you are prepared to take a part time job for a year over there to spend more time with your ds rather than the drudge of real life here, why not take a part time job here and spend more time with your ds? Or get a part time job and make up the income working nights on the days he is at his dads. Or get a term time only job so you can spend all the holidays with your ds.
There are many ways to spend more time with your ds which don't involve uprooting his life so totally.
And yes of course he is excited about it (with you telling him dad can visit as often as he wants and how fab it was when you did this at his age). The reality of it will be quite different and his dad won't be able to come every weekend and hold down his job and other commitments/life
And say he did want to come over for half a week every week are you going to pay for that? How exactly on this part time salary you intend to have are you going to pay his travel costs and at what point are you going to say 'sorry not got the money for it'...?
You need to sit down and ask your ex what his difficulties are with the decision and what you'd need to do for him to agree to it. You may not like the answer and he may well so he won't allow it.
WHat if he said only if he could come too? Or only if he could have the same in return the following year..
Also you said he's always said he didn't agree with it, so deep down you've probably always know, just haven't bothered to think about it because it didn't fit in with your plans and you've only listened to the parts you wanted to, npt what he was actually thinking and feeling about it. Now it's a reality he's making sure you know how he really feels.
He can stop you going, and it would take you many months to go through court to get any kind of decision. By which time your year would be up.
Think very carefully about what is best for your ds and both his parents here. Because it seems to me you are fixated on this dream and on what you want.
Your ds has a right to a relationship with both his parents. The court will uphold that right for him. You will be the one who ends up going to spain (alone), or not at all (assuming you wouldn't walk out on your ds for a year - which is what you are asking your ex to do), unless you can compromise and reconsider your plans to include your ex and listen to what he wants and needs here.