Children are resilient. This experience is teaching him that there are things in life that happen and must be accepted. His most recent change in behavior is possibly triggered by his disappointment and he's displacing anger as a result of it.
Behavior can be reshaped. If he is to go and be reunited with his friends, that only teaches him that if things don't go his way (and there will be many in life), that tantrums and manipulation would help him. It does not offer him the opportunity to learn self control of his anger/disappointment. At this age, children expect instant gratification.
As far as discipline, his anger/disappointment should be acknowledge (he is grieving about the separation from his peers) and dealt with. His attention ought to be redirected to other positive things that he can focus and occupy his time with. Such positive things (whatever he finds pleasant) can be used as rewards for positive behavior. Age related consequences can also be used when his behavior is inappropriate. If time put is given, it should be for about 5-6 minutes at a time. If toys are taken away, he should be told how he can earn them back.
At his age the ideas are not per se too sophisticated if explained in age related terms to him but he would do better with immediate consequence/rewards.
Hypnotherapy would not be recommended for him for this. It would be better to look into behavioral modification.
A Volcano in My Tummy: Helping Children to Handle Anger - Paperback (July 1, 1998) by Eliane Whitehouse and Warwick Pudney
My Kid is Acting Out and I'm About to Shout: Parenting Made Easy! - Audio CD (Mar. 4, 2008) by Dr. Rossi Davis