Hi Peter,
Hemp is frequently sold as a substrate for tortoises, but even though pet shops recommend it, it is not good for the reasons I gave (unfortunately pet shop staff are very often relatively ignorant of tortoise husbandry). The other thing I forgot to mention in the reasons I gave for not using it, is that pieces of hemp sometimes have sharp edges, and they have been known to cause eye injuries or internal problems when eaten (tortoises don't often intentionally eat it, but pieces of hemp can cling to pieces of food and be accidentally eaten). They live on a sandy soil in the wild and they really do prefer that in captivity as well.
The other thing is that in general it's not a great idea to have an edible substrate for tortoises. As you say, yours has never eaten it, so that is fine, but for tortoises that do eat it regularly, it can cause them to grow too quickly and develop deformed shells: the individual scutes become 'pyramided', i.e. bumpy, rather than completely smooth, and the texture of the scutes internally is more spongy than solid, and this can lead to metabolic bone disease. One of the main points to remember with tortoises is that they have evolved to grow very slowly (in the wild food is often very scarce, and there is competition from other animals). What you should be aiming for is growth of between 1g - 3g per month. That is just an average, and some months he will put on more weight and some less, but a consistent growth of a lot more than that each month will result in him growing too quickly.
That's great that you've got the UVB fluorescent light for him. I actually prefer those, as if it is too warm in the table you can raise his heat lamp higher, or if it is too cold you can lower his heat lamp, but the UVB source will stay at the same distance, which I think is better, and for that reason I use a separate UVB tube. I can't actually see the tube in your photos, but if it doesn't have one, do get a clip-on reflector for it. That will shield your eyes (the UVB can damage your eyesight, much like the UVB from the sun can do), plus it will direct up to 30% more of the UVB down into the table rather than up into the air. They aren't expensive and you can buy them at most pet shops that sell the strip lights.
http://tinyurl.com/4ysu33zd
It sounds like you are giving him a good diet. Have you been seeing his poos (just wondered why you think he might be constipated)? If he is constipated, you could try feeding him some succulents from the garden (sedum) or an opuntia cactus pad if you have one -- that often gets them going. Ha, ha, I'm probably pressing the point too much here, but having a soil-based substrate also makes it easier to see the poos and wees
Regarding his lack of interest/energy, there could be various reasons for this. It might sound odd, but tortoises do get bored fairly easily if they can see from one end of their enclosure to the other without interruption, and when they get bored they just take themselves off to hide away. Try re-arranging the 'furniture' in his enclosure-- a nice big rock that he has to walk around, one of those bendy log thingys (fiddlesticks, I think they are called) that he can walk through or climb up (they love to climb), and slide down the other side, a nice plant in a pot that breaks up the sight lines. Do you ever feed him on that little platform at the top of his ramp? If you entice him up there with a bit of food in your hand, and then put his food up there on a regular basis afterwards, then he will remember that and might well go up and down that ramp quite a lot to see if there is a treat up there for him. I always feed mine on the second level, as it gives them a lot of good exercise.
Also, if you can get him outdoors as much as possible, that is a great help and they love it outside. We can give you ideas on how to build a simple outdoor enclosure for him if you're interested.
Finally (lol, you must be nodding off at this stage in my long emai!), you probably already have one, but in case you don't, here is a care sheet for Hermann's tortoises:
https://www.tortoise-protection-group.o ... 014New.pdf and here is a link to a great site about Hermann's (Chris Leone, who runs it is probably the world authority on Hermann's -- he's in the USA and so the advice can differ in small respects from what we do in the UK, but it's all good stuff) --
https://www.hermannihaven.com/
Nina