
Have you spoken to the breeder? Different lines will probably differ with energy levels.
I've no Bullmastiff experiecne, but my English Mastiff was very similar to your pup, I even got a piccy of him fast asleep while sitting up - he didn't lay down fast enough so fell asleep as soon as his but hit the floor!
One morning he had nicked our other dogs bed during the night, he wouldn't get out of it and slept soundly (lots of snoring!) up untill about midday before he got up! (he was about 7 months old then).
Out on walks it's STILL mostly plodding, unless he's on the trail of a nice scent and he'll pick up the pace to a bit of a trot. He
can run but chooses not too, he's all about energy conservation, as a pup and now as an adult (nearly 4years). With the right motivation he can be 'actiavted' but he would rather veg out and/or sleep! Even when 'activated' by meeting a dog on a walk, he'll now only spend about 10 mins playing and then prefer to lay down & watch - sooooooo lazy!
Oh, just remebered, around about 8 months old, Buster had a very 'energetic' period, where he would actually want to run for the sake of it while out on a walk, but he grew out of that very quickly.
Busters attitude - If I've seen it before it's boring. If it's new I'll get bored with it in minutes. Why stand when you can sit? Why sit when you can lay? Why stay awake if your laying down?.....
If you are really worried then get the vet to check him over (I spoke to the vet about Buster, he gave me a knowing smile and assured me pup was normal).