Nice toothbrush, but looks like the standard button only lasts a year or two before the rubber cover starts falling apart. This is a simple replacement in TPU. I used 0.15mm layers. It locks into the original groove, but if you want to seal it up again fully you could put a small bead of silicone around the groove too.

It does have a preferred direction, so if it doesn't fit easily one direction try the opposite.