A normal healthy brain is going through the "Piaget's secondary circular reaction stage." Your baby has discovered that if he does the same thing, the same thing will happen. It's very good. It happens between 4-8 months.
At five months your little guy is simply enjoying the increased movement he is able to achieve. It's very normal for little legs and little arms to kick and box. It encourages strength. Sounds like you have an adorable bunch of energy. I wish you and your son well