It usually just shows negative, but sometimes certain health conditions can trigger a positive. I dont let my husband mess with my kits though, I just focus on my daily walking routine which my doctor allowed.
It will just be negative, so better not to waste those expensive test kits! Just focus on your own pregnancy journey and make sure to do your regular doctor visits and checkups.
My husband actually did this for a joke once and a faint line appeared, we panicked so much! But then we spoke to a doctor friend who said it's just a chemical error or some minor hormone issue. Don't worry, it's totally normal and nothing to stress about.
Honestly, it will just show negative since men don't produce the pregnancy hormone hCG. But if it ever shows positive, he needs to see a doctor immediately as it can sometimes point to testicular issues.