It is ok for a wound to feel just slightly warmer, maybe slightly swollen, and be more red then the rest of the skin, but if it feels hot, or is swollen more, I'd give the vet a call. Or you can dap it with peroxide, neosporin or another liquid antiseptic. (My vet said to use liquid antiseptic).
Dakota went back outside the day after his neutering. (His choice, once the drugs wore off, he wanted nothing to do with the inside!) I checked it a few times throughout the day, and it healed just fine.
The vet should have probably given him a shot of antibiotics anyway.