Can he pray in the clothes he slept in?
The basic principle with regard to clothes is that they are taahir (pure), unless any najaasah (impurity) has gotten onto them, in which case they must be purified.
Merely sleeping in clothes does not make them najis (impure); rather they remain pure until it becomes certain that any impurity has gotten onto them.
Shaykh Ibn ‘Uthaymeen said: The basic principle with regard to a person wearing clothes is that he (and they) are pure so long as he is not certain that any impurity has gotten onto his body or clothes. This basic principle is supported by the words of the Prophet (blessings and peace of Allah be upon him) when a man complained to him that he felt something during his prayer – referring to breaking wind. He said: “Do not stop (praying) unless you hear a sound or notice a smell.” If the individual is not certain about that, then the basic principle is that he remains pure. He may think it most likely that his clothes have become contaminated with impurity, but so long as he is not certain about it, the basic principle is that he remains pure. End quote.
Majmoo‘ Fataawa Ibn ‘Uthaymeen, 11/107
For more information please see the answer to question no. 12720
It is permissible to pray in clothes in which one has slept, so long as they are pure and cover the ‘awrah, although it is better and more perfect to wear beautiful clothes that look good for prayer. That is in obedience to the command of Allah, may He be exalted, in the verse (interpretation of the meaning): “O Children of Adam! Take your adornment (by wearing your clean clothes), while praying” [al-A‘raaf 7:31].
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked about the ruling on praying in nightclothes and attending gatherings wearing them. He said: There is nothing wrong with praying in nightclothes if they are pure, whether one comes to the mosque wearing them or not, unless those clothes will attract attention and criticism, and people will talk about him in their gatherings because of that. People should avoid anything that may be a cause of people backbiting about them. End quote.
Majmoo‘ Fataawa Ibn ‘Uthaymeen, 12/362
And Allah knows best.