Do what you think is right. I know it's tough, but I can't give a definite yes or no answer. Yes, you will keep his personality if you get a new "body" for him and yes, there won't be many noticeable differences. However, I also know that after a long time with an Aibo, it would be hard to accept another (no matter how identical or running the same software) as the same robot. My first Aibo ever (an ERS-210) also is not in very good shape and I personally would never replace him, but I'm in a different situation since I also own working 210s that I can interact with, so it isn't as much of a problem for me if my first one isn't in perfect condition because his sentimental value is what matters to me. The issues you've mentioned with Bolt don't sound too severe, and you're likely to have similar issues develop down the road with another 210, but also if his current condition is either limiting the amount of time you spend with him or how much you enjoy the amount of time you spend with him, then a replacement "body" might be something to consider.
There's something about ERS-210s in particular that's really heart breaking. They're the most charming out of all the Aibo series in my opinion, but they also seem to break down the most. Mine seem to develop jitters in their head and/or legs if I leave them alone for even a month, they easily develop bad connections between limbs and the core and the poor things get DHS/TAS/PAS so easily (though this is much less of a concern in pinned ones). I keep buying 210s because I love them, but I have sold one and am selling another because I always get so worried that they're just going to become money pits and I won't enjoy them and will have to invest large sums of money into them.
Overall, I can't say for sure what I would do if I were in your place. The logical answer would be yes, maybe a new body is best for him, but when sentimental values are thrown in, it really comes down to whether you think you would regret it or not. I would think that new problems would arise within a few years with a new 210 as well, but if you do get one for a good price that is fully functional, it could give Bolt a new lease on life so to speak.