As RoryAlsop notes, you have a problem with agreement in number between "ideas" and "is/was". But ignoring that and getting to your question:
All are valid. They technically mean slightly different things.
"He agrees is" means that he agrees in the present. "He agreed is" means that at some time in the past he agreed that this was a solution in the on-going present. "He agreed was" means that at some time in the past he agreed that this was a solution in the past.
In many cases the technical distinction would be irrelevant: if he agreed in the past he probably still agrees in the present. If the solution would have worked in the past it probably would still work today. Etc. But not necessarily.
It would be perfectly reasonable to say, "Last year Jon agreed that this was a good solution, but now he says that he has changed his mind." Or, "Jon agrees that this would have been a good solution last year, but now circumstances have changed and he doesn't think it would work today."
Without additional discussion like that, I'd take all three to mean the same thing. Unless we were talking about a case where someone might be carefully choosing his words to mislead.
(I'm reminded of back when I used to work with a Political Action Committee, and a candidate took out an add saying that he was "endorsed by XXX PAC in every past campaign". But we were not endorsing him in that campaign. By phrasing his statement to use the past tense, it was arguably true: when he ran in the past, we had endorsed him. But we were not endorsing him in the current, present campaign.)