its glute muscle not flute muscle first off. not sure if that was a typo but it is important to get the muscle correct when researching online. there is no flute muscle. glute muscle is above your buttox. when your hubbie flexes his butt you should feel it.
what happened is you did not inject into his muscle and it wasn't sterile. you may have injected into butt fat sounds like. so you have a non sterile abscess which is the lump you are feeling. the oil needs to go into the muscle. did u clean and use wipes in the area before injecting? did u draw then swap out needles before injecting? and are you SURE you injected into muscle and not fat?
you never said what size needle you are using, i hope you didn't use a slin pin. and a lot of times with the glutes if your hubbie is a big boy you need a 1 1/2 incher, 1 inch and its easy to whiff on the glute muscle.
since its warm to the touch you likely have yourself an infection as said above, its treatable with anti-biotics so no worries... your hubbie sounds like a newb and i always recommend using 1 type of AAS for newbs for this reason. and use a long ester which doesn't require frequent injections. this way you have a better chance of avoiding mistakes.
the hubbie should learn how to inject himself and not depend on someone else. and i recommend a newb hit deltoids, they are easy to inject and you can see what you are doing, or go into the pecs which are easy as well. he can even use some ice to numb the top of the skin