No suggestions here, short of hand cuffing his hands behind his back :-)
I think this is pretty normal. DD used to do that, too. I used to just wait for her to take her fingers back out of her mouth and then quickly smuggle a spoon full into her mouth. Or try and fit it in just before the fingers (or her spoon) got there...
Good luck with the solids. It's an exciting new world for them.

Sasa