How can I explain to my friend what I mean when I call him tedious?
You'll want to choose your words carefully, and use few of them. Otherwise, you risk showing him by example exactly what it means to be tedious in your presentation.
Tedious refers to something that bores you or makes you tired. You may have a tedious job or a tedious classroom assignment. You could go on a tedious trip that takes you through lots of time zones.
When you apply the adjective to your friend, you're suggesting that he wears on you. Maybe he asks you lots of questions, or talks at length about his toenails, or doesn't show much energy in anything he says or does — any of which might cause you to feel a sense of tedium.
From Oscar Wilde's The Picture of Dorian Gray:
She had proved an excellent wife to one of our most tedious ambassadors, and having buried her husband properly in a marble mausoleum, which she had herself designed, and married off her daughters to some rich, rather elderly men, she devoted herself now to the pleasures of French fiction, French cookery, and French esprit when she could get it.
And, in Anna Karenina, by Leo Tolstoy:
'I think I do. Or perhaps not...I really am not sure,' Vronsky answered heedlessly, with a vague recollection of something stiff and tedious evoked by the name Karenina.