I am not sure if it is called "he flexed his bicep"
If a child jumps on my tummy like this, I have to somehow make my tummy hard or else I will get hurt.
Do we use "flex" or "tense" to express we want to make our tummies hard? For example,
"I flexed my abs muscles when my baby jumped on it"
"I tensed my abs muscles when my baby jumped on it"
In that specific case, it would definitely be normal to say "I tensed my stomach", or as you do, "I made my stomach hard" - "flexed" would not make much sense, since the secondary meaning of "flexed" is 'bent', which is the opposite of this case.
For almost all other cases of deliberately using a muscle, it would be more common to say "flexed", though "tensed" would again be technically correct in many of those cases.
In the example where you're making your bicep as big as possible, "flex your bicep" is the most natural.
In the example where a child is jumping on your stomach, "tense" or your muscles is the most natural, and "tighten" is also good.
To "tense" or "tighten" a muscle means to engage it, usually for some mechanical purpose. If I tense my abs, I might be protecting my stomach from a child jumping on me, or I might be doing a sit-up exercise.
To "flex a muscle", on the other hand, usually means to engage the muscle in order to display it well. If I flex my abs, the intent is to make my abs look good, like maybe when I'm on the beach or being photographed.
The colloquialism "to flex (on someone)" means to show your power to intimidate someone else. It comes directly from this meaning of "flex" for display.
