IMO, a 3.5 month old puppy should not be using a halti or a correction neck device. The potential for injury at such a young age is just too likely for my comfort. The halti could end up whipping his neck around and straining his muscles and his neck is not strong enough for the choke. If you must use a head harness device then make sure you purchase a newtrix type as that works by tightening behind his head and not pulling the face around http://www.newtrix.ca/docs/index.php
. An older dog would likely be fine in a head device but a puppy is still very excitable.
I would strongly suggest
purchasing a body harness like the sensation body harness http://www.softouchconcepts.com/
or the gentle leader harness http://www.premier.com/pages.cfm?id=74
. They work by attaching in the front of the body and redirecting the pups energy around towards you. I found the sensation harness was the best fit for my large lab but I also bought the bigger version of the gentle leader and had it modified slightly to fit. It will depend on the shape of your puppy which would be best.
I did eventually go to a pinch collar as well but that was just this year when my dog hit 90 lbs and 1.5 years old. It was mostly the teenager attitude that caused me to convert from the harness as I wanted just a bit more control.