He sounds to me as though he is classic
What makes a dog "flashy" is actually a matter of genetics, not appearance. Though of course, the genes a dog has is what defines it's appearance...
To explain: Boxers (fortunately) have a reasonably simple genetic makeup as far as coat colour and flash go (as compared to many other breeds, anyway). There are two main genes at work in respect of flash - these are known as S which produces solid coat colour, and sw (the extreme white spotting allele of s) which produces areas of no pigment - flash. Every dog has some combination of 2 of these genes - since they inherit one of these genes from each parent.
The possible combinations you can get, therefore, are SS (two copies of S), Ssw (one of each) or swsw (two copies of sw).
A dog that has two copies of S will be classic. In terms of appearance, this rarely means no white at all, but more usually it is characterised by the dog having white toes, a bit of white on the chest, perhaps a white chin and possibly
a splash of white on the face/muzzle. Sounds pretty much exactly like your dog, does it not?
The dog that gets one copy of each gene (Ssw) is flashy. In appearance, he will have white feet - often extending some way up the legs, white on the tummy and chest, white on the face and possibly
a chite collar as well.
And the last possibility is two copies of sw. These dogs could be accurately described as "ultra flashy" - though we call them white. They are white over most or all of the body.
That is the simple explanation, and if you prefer a punnet square pictorial to visualise the gene distribution, take a look at www.boxerworld.com/coat_colour
In reality, it's not quite so simple since there are modifier genes in existence that influence how much S or sw may be expressed. For that reason, there are some dogs whose appearance does not clearly give away which set of genes they carry - who're somewhere between what you'd expect for classic and flashy. They tend to be termed "semi-flashy" for that reason. Of course, genetically no such thing exists - you either have a gene or you don't, you can't "semi" have it. But when it isn't clear from appearance, the only way to be certain would be from repeated breeding with a definitively classic dog (which is a rather extreme way of finding out something that isn't very important LOL). It is easier, in that case, to just call those individuals "semi-flashy".