I want him to learn that there are many ideas out there that people believe in and I want his mind open to all forms of religious belief, but not to be taught that one answer or series of beliefs is the absolute truth.
You might have trouble then...

RE in schools is either taught by the class teacher (which is what is sounds like you're saying happens at your school?) or is taught by volunteers, who are generally people from the churches in your local community. In any event, it is most likely being taught by someone who believes it is true - to the exclusion of other beliefs. This will happen no matter what religion they are sharing about, and most who are followers of a particular religion unsurprisingly believe that they are right and others are wrong - in the same way that you believe that there are many correct belief systems, and those who believe in only one (or none) are wrong.

And you can no more be asked to teach something you don't believe in, than you can expect those teachers (or that school) to teach something they believe to be incorrect.

You *possibly* would have someone who includes the message that different people believe different things, but the number of people who believe that there are different correct answers is, IMHO pretty limited. No matter what a person's beliefs are about how we got here and why, they are likely to think that they are right and others are wrong - no matter whether they're Christian, Buddhist, atheist or agnostic ...

You can change schools to one which doesn't include religious studies in their curriculum (though depending on availability of volunteers, one who doesn't have them this year may have them next year), you can exclude him from the class (which I understand from your post you do not wish to do - but at Primary School there was one girl who didn't do them in my class, and it really wasn't a big deal... it was just a fact - she didn't do them, no positive or negative value attributed to her absence) or you can let him do them, and then decide what you're going to at home.

Which seems to be where you're at now

he tells me he does believe in God- and so far that's ok with me ( i am thinking it is still on par with Santa and zombies so we can work on that later)

... I don't want him to think the religious stuff is truth

... Should I just go with the flow and wait for him to grow up a little bit before I pressure him about such things??
So from this, it sounds like at the moment (and at a young age) he has decided that he does believe in God, but you're not happy about it, and you intend to "work on that later" so that he ends up with the same belief system that you have arrived at.

And I think the big question there is whether you want him to arrive at his own decision, or whether you just want him to believe the same things you do.

If you want him to believe the same things you believe, you'd do what plenty of other parents with religious beliefs do - raise your child knowing what you believe and why, and with plenty of room for discussion. If you want him to make up his own mind, raise your child knowing what you believe and why, and with plenty of room for discussion, and also with information about what other people believe, and why, and with even more room for discussion!

But keep in mind that just as you perhaps don't believe what your parents believed - and you might have started forming your own opinions in your childhood - your child might, too.