The behaviour you are describing can easily be called "Hunting Fever". The horse is absolutely full of the joys of spring and raring to go. He's fighting you for control.
Once free he'll run until he is tired out and then some.
Until you become a stronger rider then I suggest you avoid meeting up with crowds of other riders.
A running martingale might help, a stronger bit too - say a Waterford, Cut his food back. But this is perfectly natural behaviour for a young fit horse - but ask yourself if he is too strong for you.
Be careful. This problem will not go away easily