Every base has a military clothing store so getting it there is no biggie, he just needs to have it ordered early enough. Mess dress pants need to be hemmed. The jacket will also need the braiding to be put on because the mess dress is the same for enlisted and officers, they just place different braiding on the sleeves.
Plus, you also need to realize that others will be buying too, especially if it is around the time they are about to attend a function, be it winging, AF ball or weddings. They may not have his size in stock and will need to order it in. Or they may have his size in the uniform, but not the mess dress shirt. Or they may have that all, but not the rank or the wings for the mess dress.
~~ Wingings at all of the bases, except ENJJPT are about every 3 weeks, thus a lot of people are buying their mess dress on a weekly basis.
~~ If he wants the PARTY mess dress shirt he will need to order that online and give some leeway for that to come in.
~~ Trust me I can not count how many times Bullet in his career had to run to the clothing store a few days prior because he can't find his garters, or needed a new shirt only to find out that everybody else was doing the exact same thing. By the end of his career, and even now (retired) if he has to wear his mess dress (IE wore at DSs wedding and winging) he would buy the items at least 2 weeks out.
If you know for sure he is not going to wear it than wait because he may gain or lose wait at UPT and these uniforms are incredibly tailored. Gains 10 lbs and he might need new mess dress pants.
I would also say I think the mess dress from start to finish is closer to 600. You have to buy the shirt, the cumberbund, the bow tie, the rank, the cufflinks, the insignia (wings and ribbons). That is on top of also paying for alterations.