I am starting a new postdoc position with a professor
No you're not. Not really. You're being hired as a fixed-term (post-doctoral) researcher. It so happens that you'll be associated to this Professor's research group.
Now, ok, I'm describing this in the other extreme. But the point is that a post-doc is an independent researcher, first and foremost.
who's retiring soon (<5 yrs, judging from the grad intake).
- So you don't even know for sure?
- Is his output as a researcher and relevance to your fields of interest still significant? Then it's not even clear he's retiring. Or he may "retire" to become an active Emeritus. Otherwise, ok.
I am interested in pursuing academic job and it worries me that I would not be able to count on him during my career for support after he retires.
You're implying you're not worried about his support for the duration of your appointment as a post-doc researcher. If that's the case - then you're in good shape. A lot of people don't have even that.
Am I overly worried and exaggerating the benefit of having a senior scientist interested in your work in the early stage of your academic career?
You're not exaggerating the maximum possible effect of such support, but you are exaggerating its mean effect conditioning on its being extended, and its mean extent. So IMHO, this should not be a major consideration. The main questions could be:
- Do you have interesting alternatives? Do Pros and Cons. If not, then be happy you're not busing tables.
- Will you be working on what you want to work?
- Are there other researchers around with which you can have some "cross-polination" or even proper collaboration?
- Do they pay reasonably? (If not, you might still want to go there and help organize a researcher's union; this is not related to your question but I just cannot exaggerate the importance of unionization among junior academics)
- How is the environment, housing conditions, spouse support if relevant, child care arrangements, etc. etc.
Right now, I am thinking of doing as much work as I can and using his connections to do a second postdoc with younger and active PI.
Start by "doing as much work as you can". If a second post-doc presents itself and is an opportunity for some variety, consider it - at that point.