I'm glad you have some good legal advice op.
I moved with my dc from England to Scotland without my exes permission. He tried to block me from registering them at school and was unsuccessful, they refused to get involved in parental disputes so it's not a given that they will stop him being registered.
He got a solicitor, I had messages not unlike the ones you sent, and they were used in court against him,as were any emotional outbursts and continuous calls.
The advice to get a welfare check is really poor, the advice to walk into his home and take your son is worse, going into his house without permission could potentially be a criminal matter.
As hard as it is you need to take emotions out of it and be factual, get as much factual, provable information together as you can, this will help you immensely. You could ask for statements from friends, family, teachers, club leaders etc too.
This needs to be resolved sooner rather than later, if he starts school and starts making friends and putting down roots then it's going to be harder for you to get him back, and you would be unable to do as your ex has done once a court order is in place.
Good luck op, I hope you get your boy back soon 