I say this knowing your profile says you were a college P. I also say this knowing that others may be inclined to advise differently. He should pitch to his strengths and stay within himself. (of course, hopefully, one of his strengths is velo ). He should maintain composure no matter what happens, like any good P will do. He should show all his good pitches but in the right counts/situations.
Some will say ya gotta let it fly. I am a firm believer in having the thought process that you are there because you are a good pitcher. You are the best version of yourself when you maintain the same preparation, approach and execution that you always do. I also believe that most P's have their best velo when they pitch within themselves. If a P is so inclined, the game/count situation will trigger when to reach back for a little extra (again, staying with what he usually does).
If it's like most showcases, he most likely won't throw more than a couple innings, so it's not like he is going to tire later to the point where his velo would drop. The only thing that is different is the 1-1 count. So, I would maybe put even a little more premium on getting ahead with the first pitch - make sure it is a pitch that can be thrown for a strike, either with good location or good life/movement.
All this said, I am not naive to the the fact that many P's will reach back with a few early to make sure they get that good top velo reading. Nothing wrong with picking a spot or two to let it fly but, overall, be the good pitcher that you are.
Yes, they are looking for velo. Yes, they are looking for good secondary stuff. But those are just the conversation starters. They really want to see those plus pitchability, competitiveness and composure.