Pac,
Not saying this is the case, but it's fairly common for sulcata babies to get URI. Typically Sulcata's lay large clutches and breeders will have many babies at once. When housed together, any one that gets sick can make the others sick as well.
Keep the guy under observation. You should see him perk up when the lights click on in the morning and wander around. He should be 'driven' at feeding time. Sulcatas are known to charge right over when food is dropped into the habitat. If you think that the baby is over-lethargic, take him to a vet and see if he may be sick. If so, a round of anti-biotics will usually turn the little one around. My first two torts were a Sulcata and a Leopard tortoise. Both of them wound up sick.
Yours may just be sleeping and growing, but keep an eye out. It is not normal for them to sleep all day, particularly if they are not active first thing in the morning and during feeding events.
Paul