Re: Problem student!!
True fact, not everyone in class will be a good uke for your tastes, for a lot of reasons. A dojo will have a wide range of abilities, personalities, reasons for training, sanity levels, everything. I think the mix is part of what makes for good training and personal development.
So, you've tried talking to sensei, and you've talked to the guy. Now, ask yourself honestly, have you listened to the guy, and your sensei? The guy says he's got loads of hours in other dojos. Ask him about his experience. Find out where he is really coming from. He may, if his skills really do not match what he claims as his experience, be making shit up. The question becomes "why is he making shit up?" But you aren't looking to condemn or change him, you are primarily looking to understand him. What is he looking to gain from from this apparent lie or delusion? What off the mat factors are playing into this? Is his other aikido experience exactly like the aikido training hours he is accumulating at your dojo? It helps to blend with someone before you can lead them.
It sounds like sensei is taking care of this guy. Does the poor student need taking care of, does sensei see something the sempai are missing? Why is sensei giving the guy positive reinforcement? The student sounds to me like he may have disability issues that sensei is trying to help with.
Bottom line in my universe is that as long as the guy isn't unpredictably and uncontrollably violent, he is a training opportunity. As long as sensei takes his check, he is a dojo member in good standing and is in the club, no matter what I think or what I need to change in my behavior to coexist functionally with him for a couple hours a week. The changes I am forced to make are my training. Trying to figure out why sensei does what he does is excellent training. This is the way, in my opinion, that aikido is a budo of love. Love is freaking hard work, but is always worth the effort.