How to help baby self soothe? if your going to be nast dont answer!?

my son is 7 months old. when he was a newborn i held him all the time and rocked him to sleep. then when he was a about 3 months old he didnt want to be just rocked, he wanted to be bounced and rocked, im only 5 foot and 96 pounds, so the bigger he got, the heavier he got, the harder it was for me to bounce him. he used to have to be bounced to sleep in a bouncer but he outgrew the bouncer and now the only way to bounce him is by holding him. hes 7 months old and weighs 20 pounds, its getting to the point where i can only bounce him for like 5 minutes and then my arms get super tired. ive tried to lay him down when he was tired and let him self soothe but all he does is cry. he does like listening to music while hes going to sleep, but he still has to be rocked and bounced along with the music. i have a 21 month old daughter and she was never like this, she was puttin herself to sleep at 3 months old! all she needed was a blankie and she was fine. i know i brought this on myself by holding him to sleep, but now i really need help! ive tried everything else that people have told me to try and nothing is working. someone please help! if i dont get help soon im goin to be looking like a 5 foot tall hulk lol. please help!