It means he doesn't like the idea, though it is possibly ambiguous.
"I don't think much of X" means that you think X is quite bad.
I don't think much of this book. The story is boring and the characters seem flat.
Now what he actually said doesn't make sense: Literally it means "the thing that you described only exists in small amounts" That is probably not the intended meaning.
I think we should build houses out of a strong metal that never rusts.
I don't think that there is much of what you describe. (Ie there isn't much strong metal that doesn't rust)
He probably meant to say something like
I don't think that there is much in what you describe.
Which means that the idea is not thought through or not detailed enough.
However the situation seems to be very odd. A person wouldn't just say that and nothing else. They would describe what the issues are. It strikes me as very unusual behaviour for any person just to say that, and almost unimaginable for a "friend".
So the proper answer is "ask him".