As a former 11B and current 88M, at what point can I reclassify back to 11B?

You will need one year on station prior requesting a voluntary reclass. That is in accordance with ar 614 - 200. In order to request a voluntary reclass your current MOS must be balanced and your new MOS must be under strength. In order to do a reclass without training, you will need to reach out to the Infantry MOS proponent, not the branch manager, and they will send you a memorandum stating that you are still qualified to perform as an infantryman
If your MOS is under strength, when it is time for you to reenlist you may request an option exception to reenlist as Infantry. You will still need that proponent waiver.
Third, if you have your Ranger tab, you can request for a voluntary reclass regardless of your MOS strengths, as long as you have been on station for one year.

Best way is go to Ranger school. 11b would be your secondary MOS. At that point all that’s needed is a 4187.

It would help to know what state we are talking about, but the general answer is that for E5, you need an Infantry unit somewhere in your state with an E5 vacancy that cannot be filled by promotion.
Or, when you are eligible for E6, you can submit a CPMOS waiver 4187 to compete on the 11B list, and accept an E6 11B promotion and transfer.