When a tortoise yawn is just a yawn
Tortoises yawn for unremarkable reasons. A yawn on emerging from the hide in the morning is part of waking up, and one after a large meal is common enough to be uninteresting. A tortoise may also gape briefly to adjust pressure or clear his mouth after eating something dusty or dry. The pattern that matters is frequency and timing: yawns scattered singly across an otherwise normal day, in an animal who is eating, walking, and breathing quietly with his mouth closed, are background behaviour. It is yawning that repeats in clusters, happens at rest through the day, or arrives alongside other changes that asks for your attention.
What repeated yawning can accompany
When a tortoise yawns over and over outside of waking, one possibility vets consider is that breathing has become effortful and the open mouth is helping him move air. Yawning can also appear when something in the mouth or throat is uncomfortable, or when a tortoise is generally unwell and stressed. This is general information, not a diagnosis for your tortoise: several different problems, and some harmless states, can produce the same picture. What you can usefully do is read the yawning alongside everything else he is doing, because the companion signs are what separate a watch-and-see day from a call-the-vet day.
What to check at home before you call
Watch him breathe at rest for a few minutes. A comfortable tortoise breathes with his mouth closed and no sound; note any wheeze, click, or whistle, any bubbles or wetness at the nostrils, and whether he stretches his neck out and pumps his front legs to breathe, which suggests real effort. Check his appetite against yesterday, his energy on a short walk, and whether his eyes are bright and clear. Look at the setup too: temperatures at the basking spot and cool end, recent dusty substrate changes, and whether a soak perks him up, since a cold or dehydrated tortoise behaves below par for fixable reasons. Write down what you find with times, because that record is genuinely useful if he does need a vet.
The point at which this needs a vet
Book a reptile vet promptly if repeated yawning comes with open-mouth breathing at rest, bubbles or discharge from the nose, audible breathing, a stretched neck, or food refused for more than a day or two. Do not wait out those combinations, because breathing problems in tortoises can worsen quietly and are far easier to address early. Bring your notes on when the yawning started, how his appetite and breathing have changed, and his enclosure temperatures. KinsLe's Pet Passport keeps that history, his past visit records, and his everyday details in one portable packet, so the vet examining this tortoise works from his real record rather than your memory under stress.