Sounds like he's had little socialization. Socialization is key to a healthy mastiff.
You are right in putting him in a safe place away from guests in your home because he may bite.
He needs to learn that guests in your home is ok which is going to require some consistent training and lots of patience. Find what motivates him, i.e. food/treat/toy and use that as a motivational reward for training.
When guest come into the house, while Alfred is on leash and I would recommend muzzled, tell the guests "no look, no talk, no touch." Alfred must remain by your side at all times until you see he is relaxed, therefore, learn his body language. Know when he's going into the zone that he is going to lunge. A pup that snaps without a growl/warning, can be a pup that is going to bite.
Once he's relaxed, the guests should have the motivational reward and throw those at Alfred so he knows the guests = good rewards. Again, remind them, "no look, no talk, no touch."
Keep Alfred on leash and muzzled until he has proven that he will be relaxed and accepting of the guests. He may not be "lab like" around them, but he can accept them and ignore them, if he so chooses. But, he should not be allowed to growl or snap at guests, this is what he needs to be taught. Teach him what you want him to do.
I would walk him, regardless that he doesn't like to walk. It's great exercise for him (an exercised pup is a good pup) and great socialization. Do not allow people/other pups to approach him until you've done your thing to make sure he's relaxed. Tell people who approach your pup is in training and they need to remain away from your pup. When someone runs up to me and Titan, I put up my hand signal meaning stop. I tell them that he is in training and to not approach him. Titan dislikes men and I've been working on that issue with him. Most men stay away but a brave one will walk up to us. I know Titan's body language and I can tell he's getting ready to lunge. I put up my "stop" hand and tell the man to stop because my pup is guarding me and will lunge at him. The man walks away.
Training is a lifetime commitment and pups require structure. Consistency and patience is key. If you become frustrated, Alfred will feel it and stop listening.
Hope this helps and keep us posted on Alfred's progress. If you do not nip this in the bud or keep Alfred in a separate room/crate with guests, it appears he may bite someone.