you are not in range with each other currently, you need to be on similar levels to be able to join each other in catacombs.
the range is 3 levels higher or lower the level of the creator of the game.
as you are level 10 and he is 3, this is too far away from each other. you have the following options:
wait and let him catch up till level 7, afterwards he can join your games ( and will get more exp as he is lower level )
this should work quite fast, the early levels do not take much time for a level up.
if you want to play together earlier, you may try to find a creator in between on level 7.
your friend only would need level 4 to be in range then and you can join, too, as level 10.
but note that your experience will be significantly lower if you join a game that is below your current level.
at least you can play together than and your friend can catch up during the runs, his experience should be extremely nice.