If the father of your uncle
died without repenting of slaughtering for people other than Allah and not praying during his illness, he will be regarded as a Mushrik (associating others in worship with Allah in His Divinity or worship) and it will not be permissible to perform Hajj for his father. This is because Allah (Glorified and Exalted be He) says,
But if they had joined in worship others with Allâh, all that they used to do would have been of no benefit to them.
The Prophet (peace be upon him) also said,
The step right before becoming a Kafir (disbeliever) and a Mushrik is the abandonment of Salah (prayers).
(Part No. 1; Page No. 61) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.