How to fix this leak in my on-demand water heater
To identify the source of the leak:

Dry it thoroughly.
Come back in a little bit (depending on how fast it's leaking) with a dry paper towel.
Starting at the top, carefully wipe around the blue PEX/brass fitting junction - check the paper towel for moisture.
If it's dry, move down to the brass/white plastic junction.
If it's still dry, continue moving down until the towel shows water.

When you've wiped around an area of the fitting and the towel comes up wet, you've found the source of the leak.
Is it fixable?
That depends on where the leak is.
If it's in the white plastic, that's probably not fixable (i.e., don't count on just gluing it back together). It looks very much like a bespoke piece that you will have to order from a plumbing supplier or direct from the manufacturer. Once you have that part removed, you may discover that it's just a fancy connector that could be replaced by some standard brass fittings. Your best bet would probably be to take it to your local plumbing supply house or home improvement center and find some fittings that match the threads on the heater end and on the existing brass fitting end (i.e. bodge something together that will work).
If it's in the PEX or the brass fitting, those are off-the-shelf, stock items that you should be able to source from your local home improvement center and replace. The replacement technique will vary depending on exactly where it's leaking, and would be an excellent topic for a new question with clear, focused pics showing exactly where the leak is.