I don't think he needs an "excuse" to "lawyer up".I'm thinking the reason that RT told the world the police considered him a suspect is so that he'd have an excuse to lawyer up. It doesn't look odd because he says he thinks he's a suspect. I bet he asked for his lawyer at the actual polygraph.
Anyone in his situation is naturally going to come under suspicion, being the last known person to be with the missing person. You've only got to read some of the comments here and elsewhere.
It would be really stupid not to get legal advice.