He should have spoken with a lawyer before talking to the FBI.

You should ALWAYS talk to a lawyer before speaking to any law enforcement.

You need to say "I am exercising my right to remain silent", then shut up.

And based on the second article I posted, you also need to make sure you don't look uncomfortable while stating that you are exercising your right to remain silent.

That's not how I read it at all. The "uncomfortable" is only if you remain completely silent.

Of course, but he didn't have to say anything.

Lying and answering truthfully are not the only options.

He could simply not speak to them.