Postdoc with a professor retiring soon I am starting a new postdoc position with a professor who's retiring soon (&lt;5 yrs, judging from the grad intake).

I am interested in pursuing an academic job and it worries me that I would not be able to count on him during my career, for support after he retires. 
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?

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.

Any advice would be appreciated!
You are overthinking it and maybe trying to plan too far in advance. Your current prof, after retirement, will most likely still be able to help you. He will still have his contacts and may still give you some, if less, feedback on your ideas. 

You are right to maximize what you have now. But as time passes you will have the opportunity to fine-tune your future. You will know more in a year or so. You can explore with your current prof what he intends after retirement. It may be retirement in name only, perhaps. You can also get connected to his network as you work along, so that his direct presence is less important. 

Far worse if someone leaves suddenly for any reason. That can leave you stranded. This situation seems well controlled with a lot of planning time.