He got blood on his underwear and it is hard for him to change it

What everyone who wants to pray must do is to purify his clothes of any impurity that is on them, because Allaah says (interpretation of the meaning): 

“And purify your garments”

[al-Muddaththir 74:4]

And when the Prophet (peace and blessings of Allaah be upon him) was asked about praying in a garment in which a woman had menstruated, he ordered that it be cleansed of impurity before praying in it. Narrated by al-Bukhaari (308) and Muslim (361). So you have to purify your garment or change it before praying. You can ward off hardship by putting a pad or piece of cloth to stop the blood spreading, then when you do wudoo’ you can change this pad or cloth and wash the part of your body that the blood has gotten on to. If that is also too hard for you, then there is nothing wrong with your praying in this garment. 

Shaykh Ibn Baaz (may Allaah have mercy on him) said: The sick person who is affected with urinary incontinence and does not recover with treatment can do wudoo’ for each prayer after the time for it has begun, and wash off whatever has gotten onto his body, and set aside clean clothes for praying if that is not too difficult for him, otherwise he is forgiven, because Allaah says (interpretation of the meaning): 

“and [Allaah] has not laid upon you in religion any hardship”

[al-Hajj 22:78]

“Allaah intends for you ease, and He does not want to make things difficult for you”

[al-Baqarah 2:185]

And the Prophet (peace and blessings of Allaah be upon him) said: “If I command you to do a thing, then do as much of it as you can.”  

But he should take precautions to prevent the urine spreading on his clothes or body or the place where he is praying. 

End quote from Fataawa Islamiyyah (1/192) 

And Allaah knows best.