Im a new DM having a heap of fun running LMOP for my partner, her 13 year old autistic son (X) and a couple of his friends.

As X has been playing through and finding his feet in the game, he has become more & more ingenious and his imagination has some awesome ideas.

However, most recently he has been suggesting "we have all we need to make a gun!" or "we could make a grenade!" or various other suggestions about modern ballistics or projectile weapons.

This is LMOP and the playing party has already immersed themselves deep into the lore and environment of the Sowrd Coast and Fae'Run.
Indeed some dropped hints and plotline steering has peaked interest in my intended next campaign - Out Of The Abyss (Nezznar has some connection to Ilvara)

With that said, as DM with support from my partner, we have enforced the "No modern weapons" rule to our table.
Ive clearly explained that if his Dwarf only has a BattleAxe and I turn up with an AK47, its hardly fair is it? 
He took that and all was good.

But then...
I gave them a Mimic to deal with in Thundertree to spice things up a bit.
They killed it off quickly and X dealt the final blow, providing in glorious graphic detail how he sliced off the purple psuedopod, and slung it over his shoulder as a trophy.
Later while resting he then hung it over a branch near the campfire, dried it out and again, with a 10 minute detailed explanation, ground it down into a powder using two nearby boulders.

I cannot deny this, as the whole point of us playing D&D was to encourage some social interaction with his friends, get him away from his XBOX for a while and use his imagination.

So he's now the proud owner of a 1lb sack of Mimic Dust.

SInce he was trying to see if it could be used anyway in combat, I've already ruled that it is neither magic or poisonous. It is just the ground ashes of a former mimic.

But X is a clever lad.

He has suggested that if it is ash, and therefore flammable, he could throw it towards something, then have a flame ignite it - making a spontaneous combustion explosion like a flour or milk powder cloud.

This is a 13 year old lad talking about and demonstrating his amazing knowledge of science and physics. I can't knock that - I want to reward his initiative.

So Im thinking now that I will allow this to be used - just once - after he has attuned to the idea a little.

But I'd welcome some guidance on how to use or allow this improvied weapon and what damage it might cause?
Is it just an example of the Bomb stats/rules from DMG 267 or is there an alternative?

Also, does it set a dangerous precadent that he can use explosives in this setting?
Im worried that he may spend the rest of the campaign looking for components and materials to create greater bombs or weapons.
How would you handle this?

Thanks in advance for any assistance.

A little bit of knowledge is a dangerous thing.  The fact that such an explosion is possible does not mean that you can easily recreate the conditions necessary for it.  Causing a dust explosion in a non-enclosed space is incredibly difficult to do in a combat setting.  If you're going to allow it, I would suggest going with the bomb stats and rule that it's some property of the mimic ash that allows it to work, allowing you to easily prevent future abuse.

Thanks for your reply :)
I like and agree with the caveat that it's a special property of the mimic dust. This should help.

You may also want to simply use a spell to simulate the effect and that spell be no greater than level 1

My choice is a modified Earth Tremor

If it is mimic 'dust' then let him stumble upon a recipe for somethign akin ot Dust of Tracelessness or Dust of Sneezing and Chocking where mimic dust is part of the recipe or, if you feel like giving out the ability to make a mgic item, have it as the component to create a Hat of Disguise.