我爷爷的全身很疼，他不能去游泳: Is this sentence grammatically correct?
他对我说：“我爷爷的全身很疼，他不能去游泳”


的 in 我爷爷的 (My grandpa's) is a possessive marker, therefore follow it with a noun '全身' is  grammatically correct. But 的 can be omitted here

我爷爷的全身 = 我爷爷全身

Native Chinese would write "我爷爷全身[都]很疼" [所以]他不能去游泳” (My grandpa's whole body is [all] in pain, [therefore], he can't go swimming)