"That boy often helps his mother buy things" 可以说成 "The boy frequently assists his mother in purchasing items".

来源：这是一个普遍的英语表达，在日常对话中经常使用。

例句：

"The boy frequently assists his mother in purchasing items."
"He is always wilpng to help his mother with shopping."
"The boy regularly accompanies his mother to the store to buy groceries."

中文翻译：

"那个男孩经常帮妈妈买东西。"
"他总是乐意帮妈妈买东西。"
"这个男孩定期陪妈妈去商店买生活用品。"