The second sentence makes more sense, because 'suitable' pertains to 'post',
which is why they should sit closest to each other in the sentence.
What is a 'suitable man' is really the question? Would anyone understand that?
Context is, of course, everything. So, just maybe this makes sense—in a given context.
If you consider the second to have parts omitted (text simplification), in other words,
"He is a man who is suitable for any post." it becomes visually more clear how the
two sentences differ.
Having said that, why not say "He is a _______ man, which makes him a suitable candidate for any post."
But then again, I think one should avoid using the verb BE in relation to people. Use active verbs
instead. "He's a _______" puts him in a box. Instead, "He works as a _________" and "He manages..." sounds a lot more empowering.
Hope this helps. (My apologies, if my answer is round-about-like, but I'm new here and it's my first answer on this platform—but there's more to come, so you're welcome to follow me.)