I think it will be ok to leave him for that long, esp if it's only 3 days a week. Not ideal obviously, but these things happen and we just have to do the best we can when we do. I certainly think it is better than any alternative. It would be a good idea to get another cat for him for company, esp if you think this may be more than a temporary thing, but the problem with doing that now is you can't be around to do introductions properly. If you have to keep them separate then you just have 2 cats who are on their own all day and you're probably going to be stressed enough by all the travelling etc without having to cope with the potential problems that can come with introducing a new cat. Of course they may get on great really quickly, but if they don't you could just be making life very difficult for both yourself and Sam. Maybe after you've been doing the commute for a few weeks you'll have a better idea of a) how Sam seems to be coping and b) whether you think you could cope with the stress of introducing a new cat while you're doing the long hours.
I'd get some new toys for him - perhaps one new thing each week - and pay particular attention to his environment on the days you have to leave him for a long time. Then when you leave him rearrange his toys - put cardboard boxes in a different place for example - and get out the new toy for that week so that it's all a bit different for him. You can leave food in automatic feeders and those food balls that they push around the floor to get the food out of. You can get the latter as part of a timed dispenser that will releae them at set times so that might be something to look into.
At least at 1 he's passed the very young kitten stage. How lively is he? Tbh he probably will just sleep while you're gone. The hard part will be when you return home tired and he wants to play! Do you have a friend, neighbour or relative who could pop in during the days you're gone to give him a bit of company?
Try not to worry too much. It may not be an ideal situation but I'm sure Sam will cope. Just give him loads of love and attention on the days when you're not gone so much.