In order to access the same catalog, you both need to share the catalog after doing any changes. Images should also be shared and kept in the exact locations.
This would be one way to accomplish your desired workflow.
You may also consider the another method.
As you have stated, you wish to help him to edit, I would suggest you to work on a set of images and share the catalog with him.
He just need to merge the catalogs in order to see the changes in his own catalog.
This sharing process could be greatly enhanced through the use of Lightroom Mobile. However, that would require at least one of you being subscribed to the creative cloud, and the other one could contribute to the editing through Lightroom Mobile. But if you are like most of the users who contribute this forum, Lightroom Mobile and the creative cloud are pretty much out of the question.
You could exchange an exported catalog with Smart Previews-
1. You select some images and do File > Export as Catalog. Check (Smart Previews), but not negatives (originals) You will generate a catalog and folders with your work done and the smart previews.
2. Zip the Exported Catalog and Smart Previews folder.
3. Send the zip file (dropbox or email)
4. Son unzips and opens the Exported Catalog.
5. Son edits images, then repeats steps 2 & 3 to send it back.
6. You now use File > Import from Another Catalog to bring the extra work back into your catalog.
Thanks for the advice. We are both on Creative Cloud (both subscribers to the photog. pkg). I thought of using either LR Cloud or LR mobile, but when I first looked at them, I couldn't see how to move photos between collections, create a new collection, etc. I now see that I can do that on LR Cloud, so maybe I'll try working that way to edit down to a useable set, then get him to export a catalog when I want to actually do any of the developing. When we get to creating a book or slideshow, that will present another collaboration challenge, but I'll cross that bridge when I come to it.
You will strike limitations with the "Export Catalog+ Smart Previews" sharing method!
To export derivative images at full size, and create slide shows, books, etc- LR requires the 'original negative' image files. That's OK for you (with the originals), but not possible if you have only the smart previews.