Re: training troubles
The concept of blending is especially important in aikido. But, one aspect that is commonly overlooked is that your training partner often does not meet you exactly in the middle... With practice, you will become better able to adjust to those "off-center hits" that happen when one partner or the other is a bit too exuberant or too passive. Let's face it, aikido could not be considered truly martial if you could not prepare yourself for an adversary who is strong. So, you might find yourself better able to accept this individual and his "differences" if you use it as a means of pushing the envelope of your own training.
That said, if you have serious concerns about his injuring you, and you feel certain that your instructor will not help you correct the situation, then you need to seek a different alternative, and possibly look to train elsewhere.