It is an individual thing among males, but to be safe, I would ask your vet when s/he feels comfortable with neutering. Some want to wait until 6 months of age, but others will do it as soon as a certain weight is achieved and the testicles have descended. Call around in your area. Also, there is a lovely list of Low- or No-Cost Spay/Neuter clinics listed by state at www.lovethatcat.com/spayneuter.html
. There may be a clinic nearby you which can provide early neuter at a lower cost.
Best of luck,