Is it cheating if he’s on his mission?
Is what cheating?
If you two have some type of, “I’ll wait for you and never date anyone else for 2 years” and then you do…yeah it’s cheating. But why put yourselves in that situation? Say, “Have a great mission Elder So and So, we’ll both be different in 2 years, let’s see where we are when you get back.” This keeps your life moving forward and him focused on his mish.