I agree it is nice to buy a keepsake type item for the child. However, at our church they always give the kids bibles - so that might not be wise.
Maybe even a small pin that he could wear on his lapel or on a tie later on as he gets bigger. You figure he won't dress up often, but when he does he'd have something special to wear & would remember when/who got it for him.
Maybe a cross to hang or put in his room - somewhere that he could stop at night and say prayers. My son kisses his cross & says prayers there every night, of course he is 3.
Money is always nice, but I prefer to give a gift too.
You could do a small dish/box for misc. knick nacks from Things Remembered or that type of store w/ engraving or not.
I'm sure he could use that in his dresser for years to come.