How complex are the objects he can form? Simple solids of basic shapes or complex mechanisms made from a number of different materials including alloys, compounds, mixtures etc. What about molecular velocity - the difference between a hot object and a cold one.
This then begs the question of how he knows enough about what he's making to make it.
If it's just a simple shield, he could zap up a chunk of solid matter but will it work? - molecular density, structure etc all play a part in it.
For a more complex device such as a firearm that contains many moving parts and different materials -
some of which are tempered differently to one another - he would need a lot more knowledge of what he was doing. Even more knowledge is required for it to be loaded with bullets that will actually fire.
Is there a shadow power at play - "scanning" - that enables him to analyse objects he's seen (IRL, not just a picture or televised image) and therefore enable him to recreate them later?
He would not have to be consciously aware he has this ability or consciously do it - in fact, it could have been active all his life, instinctively scanning everything he encounters - but the upshot is that when he suddenly needs a shield his mind is able to provide all the parameters the "Create Object" power requires to function usefully - solid chunk of dense material with a rigid molecular structure suddenly appears.
Just a few thoughts, anyway.
Edit:
Where does the energy for all this come from? Wayne suggested being able to create food - don't know if you plan for your character to be able to do this or not but it certainly raises an interesting issue: How much of the character's own energy reserves are taken up tearing down matter (presumably down to subatomic level) and building it back up again? How hungry - in need of replenishing his own energy reserves does he get after doing this.
Could he manufacture food? Would the amount of food he could manufacture supply sufficient energy to compensate for what he lost manufacturing it? Conservation of Energy would suggest not...
Even if his power, once activated, was 100% energy efficient and perfectly balanced out the Matter to Energy/Energy to Matter equation while turning air into a shield, he's just expended energy of his own in order to do so. Is that expenditure proportionate to the complexity of the change and or the mass of matter and energy states involved?
Air into a simple shield of the same temperature as the air vs air into Steak, Eggs 'n' Chips - nice and hot
Well, according to the Create Object power in the DC Adventures book (page 93), he can only create simple geometric shapes with no moving parts. However, there is an extra called Precise which allows for objects with moving parts and even more considerable detail, which is still left up to the DM, which I gave him. But food is matter, so since I haven't played him, but would like to, it's up in the air if I could do that or not. I would very much like to be able to do this kind of thing.
I was trying to mold the power after a power from the old MSH book.
With original MSH rpg, he had the power of Artifact Creation from the old Ultimate Powers book, which allowed him to create any objects that he could imagine, even imaginary objects such as tricorders. However, he could only create a number of ounces up to the rank number, and each ounce drained one health point.
And it's not energy to matter, it's matter to matter. Which means as long as there's particles around, he can create it.