3.5 is definitely too young for a regular season game. Take them to a pre-season game, so it's about half full (or less). Then when he gets squirrely he can climb around your section because it will be pretty empty in the second half. You can also take him to the concessions frequently and not have to worry that you're missing much.
Every kid is different. I took my kids when they were 6 & 7 to a regular season game and they each got to bring a friend, and they had no problem staying for the whole game and were mostly into it.
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