The child's behavior had improved recently, but yesterday he started regressing again.

regressing 是动词 regress 的现在分词形式。regress 由前缀 re-（表示"向后、往回"）和词根 gress（来自拉丁语 gradus，意为"走、移动"）组成，字面意思是"往回走"，引申为"退步、倒退"的意思。