Is it a good idea to have really busy advisor?
In general, the answer is no. You normally want an advisor who can spend time and effort with you. This becomes more important at higher levels of education. A too busy doctoral advisor, for example, is deadly. But the specific case may not match the general case. 

Much will depend on the nature of the thesis you need to write. How "far out" from your normal studies is it? How well do you already understand the problem and see a path to its solution? A deep research topic in, say mathematics, when you don't yet know the required basics, might be too much. A philosophical paper (in any field) in which you give evidence for and against some point of view might be just fine. 

At the undergrad level, having an advisor who is simultaneously doing a PhD might be an advantage as he is connected to a lot of ideas. If you are depending on the advisor for ideas for the thesis this could be a good thing. 

How self motivated are you? How dedicated and hard working? You won't get a lot of face time or long answers to email from such a person. Is that ok? Can you push through with only minimal advice? Say in the time it takes to ride a couple of floors in an elevator? Some advisors are good at saying the right thing and saying it quickly. 

You write that he works in an area that you are interested in. That is important. Maybe very important. What are your other options? Either for advisors or for topics. 

Ultimately, though, it is is a tradeoff of many factors that only you can judge. You might make the right decision or not. Try to not make the worst decision.