Not exactly sure what to do here... I have a 3 year old who seems to like the softer bits (rubber and happy mouth type) However I've been noticing that with the rubber Dee I have him going in now he chews the rubber off, doesn't swallow but i've had to cut pieces off (so he doesnt). I'm worried that if he does this in a happy mouth that the rough edges will get too sharp and poke and stab him in the mouth.
I've had him in a normal steel bit but get much better results in a softer bit.
You may just need to lower your bit a hole or two. In a different position, he may not be able to chew it as easily (and may not want to). In general, I'm cautious about rubber bits - the extra thickness in the mouth (especially if the palate is low and the tongue is thick) can be uncomfortable for some.
If it's a fat rubber bit, might be too much in his mouth right now. If you go w/ a happy mouth which my young ones like, once they chew on them and the bit gets bite marks on them, I wrap them in latex bit wrap. Just a couple inches wrapped around and voila, they can still chew but it's soft for them.