I had him in freshwater for about 2 weeks and then he got whitespot which was a pain because it is very hard to treat puffers so instead I decided to slowly over 2 weeks bring his salinity up and have now had him at 1.010SG ever since and he absolutely thrives in it, has grown about half his original size again.
Mine absolutely loves freshwater shrimp he actively hunts them and normally devours 2-3 in about 10 minutes depending how smartly they hide.
I do think they do better in a brackish environment but also think they will live quite happily and healthy in a freshwater tank