Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, answers: “The legal status of someone who does not pray is controversial among jurists. If someone does not pray and denies the obligatory character of Salah, he or she is not considered a Muslim any more according to the consensus of scholars.
As for someone who misses prayer out of laziness, it is a controversial issue among jurists. The view believed to be the most correct is the one adopted by Imam Abu Hanifah, may Allah bless his soul, that he or she is a sinful Muslim. He or she is committing a grave sin, but we still regard him or her to be a Muslim as long as he/she believes in Allah and bear witnesses that Muhammad is His Messenger.
As far as performing Hajj on behalf of someone who does not pray is concerned, if we know FOR SURE that he or she doesn’t pray and that he/she doesn’t regard Salah as an obligation in Islam, then it is not allowed to do Hajj on behalf of him/her. But if we know that he or she doesn’t pray but we are not sure whether he or she regards Salah as obligatory or not, then it is allowed to make Hajj on his/her behalf.
As long as you said that you are in doubt about whether your uncle used to pray or not, we would say it is allowed for you to do Hajj on his behalf, as doubt cannot be taken as a basis or a ground to establish ruling in Islam.”