Diabetic father, says that his insulin shots no longer have an effect when he injects on his abdomen

he tried injecting the short acting type in his thigh and it worked there, he is wondering if its okay to inject the long term insulin there, he says he's worried because its a gel and is supposed to slowly secrete the active insulin, therefore it should be injected into fat, and his legs don't have much fat at all, the question is: would injecting the long term insulin into his thigh affect how it is normally supposed to work? considering that his thigh doesn't have much fat in it?

thank you in advance