My developer tells me that support for Beans and further development is being withdrawn.
As my site is built with the Beans framwork, he says that my site, which is pretty large will need rebuilding wioth something else soon.
Is this correct? Or could he just be angling for more work?
Sadly, you're developer is correct. Beans hasn't been updated since 2018. The developers who created Beans have moved to other projects, such as Genesis. You should consider rebuilding your site with Genesis or another similar framework.