At his age, all you need to do is talk with him about it... openly without judgment/criticism/scolding etc. Just talking about it.
Maybe if you ask him, he will explain and/or have a reason?
You are very lucky that he has never had a "tantrum" until now. THAT is unusual. But that does not mean he is "abnormal." And NO NO NO "tantrums" are not only for little kids.
To keep it in perspective... even Teens have tantrums too. Mostly, kids get tweaked when they hit certain age junctures or are on the cusp of another age etc. Growing pains.
Some kids are also not expressive... so they keep things all bottled up inside them. NOT good. Then they get frustrated and don't know how to express themselves, or they think they have to be strong and silent. Then they erupt in other ways.
Tantrums are an expression of frustration/inability to express themselves, or the child not knowing how they feel, or them not receiving understanding or validation, or some kind of stress or problem occurring (big or small) at home or school or with situations, or a change in routine etc.
So thus, just ask him and see what his thoughts are.
And by the way, "tantrums" can still occur even at 6 years old.It depends on their emotional maturity as well, not just their age.
My daughter, when she was in 1st grade, had a classmate that "tantrumed" still, for example.
A great book is: "Your 4 Year Old" which you can get from www.amazon.com
It is a book about each age-set and their characteristics. Although written years ago, it is still Very pertinent and timeless.
Or, if anything has changed in his life in the past 2 weeks, then explore that. Maybe he is reacting to it, thus tantrums.
All the best,
Susan