If the horse is a full five years old, his central incisors, both top and bottom,'should be' fully in place. This appears to be the case with the bottom ones(notice that the centrals and middles are clearly larger, 'wider', permanent teeth-which have MUCH more of a 'root', though you can't 'see' that part- -while the corner incisors, both upper and lower, appear to still be baby teeth, and 'should be' being shed SOON!) Minis shed their 'baby' teeth later, in general, than fullsized horses, but it shouldn't be TOO delayed, else you have problems.
I'd bet on one of two things...that his upper central 'baby teeth' may not have previously been shed, and were 'knocked out' by the fall...the abraded look of the gums around the barely erupted two upper central teeth is a big clue that a tooth has recently come out of that location in the gum. If so, the fall may have actually been a blessing, enabling the permanent teeth that were 'stuck' behind the unshed baby teeth to now go ahead and come in as they should! A horse 'should' shed his central baby teeth FIRST, so this horse would have been quite a bit 'behind' on those upper ones, as he's clearly already shed, AND, 'grown in', the lower centrals and both upper and lower middles.
It is always a good idea to check in a young horse's mouth now and then, to check if it is 'on schedule' for the shedding of the baby teeth, both incisors AND molars(it may take the vet to check the molars, as they are so far back in the jaw, and very hard for the layman to be able to tell!)
The other possibility, IMO, is that what you see are his permanent central incisors, but they were 'driven back into' his gums by the fall. This I think is unlikely, but could be a possibility. Just keep an eye on him, see what happens--if he had 'baby teeth' that were knocked out by the fall, the 'underlying' permanent teeth now visible should be growing into their proper place. If you have any doubts, I'd suggest consulting your vet.
Margo