Squishy, growing and itchy lump? yes a trip back to the vet that saw it first time is in order as it could be something more serious than just a cyst.
At the same time please get him neutered
by your vet (or at a spay/neuter clinic if there's one in your area). At same time vet may do needle biopsy on the lump. At 9 mos. your boy is sexually mature and you sure don't want him "marking his territory"=spraying urine (inside your house/apt.) like a lot of males have the urge to do, as you won't want your place reeking with a pungent stink, and wondering where it is. All the best!