He looks gorgeous. Often they do have less hair behind their ears - so if that is the only place it seems thinner then I dont think you have a problem, but I am sure you will be able to have a good look at him when he has settled down a bit. From what I can see in the pictures he doesn't look particularly thin haired so hopefully he will be ok.
The skirt on a long haired hamster can look a bit 'woolly' but I am sure a good diet and grooming will help. If he hasn't been regularly groomed you may find that on grooming he loses some of his skirt and if it is matted then I am afraid that is somewhat unavoidable I think.
Syrian hamsters don't need/enjoy sand baths like dwarf hamsters or chinchillas. In my experience they don't seem to know what to do with it! I have kept some of my Long haired syrians on chinchilla sand, but they never rolled around in it so I wouldn't worry too much about that - just give him a good diet and as soon as he is comfortable start the grooming