Break the crayons so small he can't fist grip.
Golf pencils (do the same thing as a broken crayon, makes it so you have to have a proper grip).
Handwriting without tears (10 minutes a day...lots of information about grip)
Playdoh and other arts and crafts activities to strengthen hands.
Pencil grips that you can find online from OT websites.
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That said 3.5 is young. But I would incorporated some of these things whenever possible and all of them at age four if he still hasn't fixed his grip.
My son paid the price for no one fixing his grip until first grade....I tried, believe me. But he wasn't always in my site when doing paper activities and there is so much more to life at home than doing things with a pencil. Finally the first grade teacher made it her mission and it wasn't long before he figured it was easier to comply. His handwriting is still so bad he was evaluated by an OT this year (4th grade).
