If I Accidentally rejected Roderick's special ability while trying to switch to another character and assign the skill to someone else , will he ever offer that skill again? --- Jesus said to him, I am the way, the truth, and the life: no man comes to the Father, but by me. John 14:6
It sounds like you're playing the original tactics ogre, in which Roderick appears in the hell gate to give you special abilities. That actually doesn't happen in this version of the game, instead every character can learn skills for each weapon type.
Back to the question : I haven't played the original in while but I'm fairly certain that he will only make the offer for each skill once, so you're out of luck if you don't accept it for anyone.
To be honest though, most of the skills aren't anything special. Fudo is really powerful and you should definitely give that so someone, and rage is pretty useful on an archer. Relic and Rampower are semi-useful but their effects can be reproduced through other means, so you don't really need them.
The other skills tend to either be really dangerous or underwhelming in terms of their power, such that you're probably not going to use them over a normal attack most of the time. So unless it was Fudo that you gave up, you're really not losing much by skipping one.