Hello & welcome Cris
It seems to me that you are so emotionally involved in this that you cannot be objective.
Your ex seems to be following you and phoning you all the time; he was possessive and jealous; you only broke up in December; you are not yet over him.
What does this mean, exactly?
Let's consider the ex:
Are you 'still not over' him, because you still care for him?
Is this relationship still 'active' in some way ~ since you say that you are not over him & he is obviously not over you?
Do you want him back?
Are you flattered by his messages and his constant attention, or do you really feel as if you are being stalked?
What is really going on between you?
Is it completely over?
If so, have you made this completely clear to your ex?
Does he think that he may still have a chance with you?
The new boy was respectful of your relationship, when you were with your ex, but has now made it clear that he is interested, but obviously does not know where he stands, because your ex is always around.
What worries me, slightly, is that your ex sounds as if he may have control issues, but so does this new boy ~ telling you how to behave and what to wear. On the other hand, I think that he may just want to date you, and feel that he cannot pursue this, while your ex is always in tow.
Of course he hates your ex. He seems to fancy you and he cannot be around you without him popping up at every possible moment.
Have a good think about this.
Is it over with your ex?
Are you really interested in the new boy, or is this a rebound thing?
Try not to lead either of them on.
You used the word 'stalk' ~ is your ex likely to cause any trouble? If you tell him, clearly, that it is over, will he understand or could there be problems?
If it is over, then he needs to know this, and understand, that he cannot follow you any more. This is for his own benefit, as well as yours and that of any new boyfriend.
If you think that this could become serious, then tell your parents and / or tutors. You don't want anyone getting physically hurt. If this really is 'stalking', and he could become violent, then you may even need to tell the police.
Only you can know whether or not you want this new boy to be your boyfriend, but he cannot get involved with you while you are still involved with the ex, so this needs to be sorted out.
Just be careful not to jump from the frying pan into the fire.
