I am not sure how relevant my experience will be as public schools may have changed greatly since the mid-1990s when my family moved to Barcelona from Albuquerque, NM. My children were 6, 11, and 13 at the time of our move. The older two adapted well. The youngest one had a more difficult time, but he is quiet and not a complainer so he didnt say much. We moved to Castelldefels just outside Barcelona because I was concerned about traffic. I enrolled them in public school there, in which Catalan is taught as the first language and Spanish as the second. My older two children could speak some Spanish and adapted well, but after a couple of weeks my daughter told me, "It is so sad seeing Bjorn all by himself at patio (recess)". I immediately made arrangements to see the school administration and they made efforts to integrate him, in part by getting him together with other immigrants (kids from other parts of Spain wete refered to as immigrants). Things went better, he made friends and we would take him to his friends homes so he could play after school. He attended first grade there and learned Catalan and Spanish. It is now 20 years later, my children all speak Spanish fluently, are world travelers, and in the case of the older two, the ability to speak a foreign language has helped them in their careers. My younger one is also doing well and after initially studying one career, is now finishing his BS in Mechanical Engineering. The only thing that I would do different would be to move to a good neighborhood in Barcelona rather than Castelldefels.
Suggestions based on my experience:
1) Choose a good neighborhood to live in, but preferably one with some foreign students so your son has some friends from day 1 that speak English. Get guidance from your workplace or university. Also on housing. You may be able to avoid the real estate agent fee which is typically one month's rent.
2) A main difficulty is that very young Catalan children are more comfortable speaking Catalan than Spanish. Find a student to help your child learn Catalan after school hours, and take a class yourself. If you are moving next summer get Catalan and Spanish tapes to start learning here.
3) From day 1 make meetings with the school administration and ask how they can help integrate your child. Also speak with the teacher. We found one teacher that took special interest and that made a lot of difference. Generally, Americans are liked in Spain (in spite of Trump) so making friends won't be hard, but there are differences in Spanish customs. Being from Argentina will help.
4) Buy an electronic notebook or tablet for your child and install games that they like, some in Spanish or Catalan for times when he/she is alone. My son spent a lot of time on my laptap playing microsoft free games.
5) Socialize. At first it will be difficult to make friends (easier in other areas of Spain), and you may meet unpleasant people at times. We had great friends. My daughter still maintains contact with a friend from Castelldefels.
6) If you are returning to the US, you may need to help bring your child up to speed for schooling they have missed here. In the case of my son, having missed first grade, he was behind in reading in English, and a teacher had him stay after school to teach him. In my case, I neglected to help him catch up. Have you child read to you in English starting a few months before your return.
Hope that helps.
Carl