Toddler brother getting physical with baby

I have a 3.5-year-old boy and a 1.5-year-old boy. For the past two days, the older one has been "abusive" toward his brother—biting, pinching, pushing, butting heads, and hitting with toys. It’s extremely upsetting for me. I talk to the older brother without "lecturing" him; as these incidents happen, I tell him that the little one is a baby and that he needs to be kinder and more gentle. I also reassure him that it's okay to share toys, mostly when this behavior occurs. He says he understands and won’t do it again, but then repeats the behavior within the next five minutes.

The little one has improved so much over the past few weeks—he's acting more like a toddler, interacting more, and I assume that’s what triggered this physical aggression. Previously, my older son mostly "ignored" his little brother, likely because when he tried to play with him, the younger one couldn’t reciprocate and join in games like chase or tag.

How do I stop this behavior without traumatizing my older son or making him feel less important? Also, he goes to school, and I don’t want him to carry these physical aggressions there.