@Dimitris You shouldn't focus too much on the broken "il est retard". Just assume the OP forgot the ending "é".
You shouldn't focus too much on the meaning "he is mentally deficient" because it is anecdotal here and the context should clearly sort out what is intended, not to mention it has no sense if "il" isn't a person but a train, a plane, a training, a Tour de France, whatever.

You should state that the usual meaning of "il est retardé" is "he/it is delayed" and compare it with "il est en retard". This is the interesting point of the question.
The difference between "il est en retard" and "il est retardé" is that in the former case, the expected time is over, or will be necessarily over (it's too late to be on time) while in the latter case, the expected time hasn't yet arrived. The person or object delayed might even be able to catch up the delay.