My baby is weaning off previcid - is there a concern for his reflux to come back worse?

No. The fact that your baby is successfully weaning off the Prevacid (lansoprazole) means that the reflux is most likely improving. If you stop the prevaci the reflux will either be resolved or no worse than it was when you started the prevacid (lansoprazole).
In some babies,yes. Most parents won't notice a change , but some might.
No. If you wean too early the baby may resume with reflux, but it won't be made worse by weaning. Watch http://www.Youtube.Com/watch?V=qetnevh4fzk for more info on gerd.
Not Worse Than Start. If the weaning is unsuccessful, and it may take several weeeks to know, the reflux will be no worse than that before the treatment.