Blowjob help 😩

Today I gave him a blowjob that lasted for quite some time, maybe almost 15 minutes. He didn't have much of a reaction but told me I was doing good, telling me to try going slower and then faster. Finally gave up bc my knees were dying (I got bad knees rip) but I was wondering if y'all had any suggestions to improve? I try using my tongue but he says he can't feel it at all. Please help !