I bought Halo's backpack when she was 16 months old, and had her wear it empty for two months. At a year and a half I started pretty light, with just a 1/2 liter of water in each side, and then built up to more weight. Properly conditioned, an adult dog can safely carry as much as 25-30% of their weight, but mine are usually in the 10-15% range because we only do day hikes.
With the Ruffwear packs there was enough of an overlap between sizes that Halo can wear either a small or a medium, and Keefer can wear either a medium or a large. I chose to go with the smaller sizes since the bags are sized accordingly, and I don't need the extra capacity.
Keefer was an adult when I bought his pack so he was easy - his girth is 33", which put him 3" under the maximum for a medium (26"-36"). With Halo I did have to guess, but her growth had already slowed down quite a bit by then, so I felt safe going ahead with the small - her girth was 28" at the time (no idea if she's any bigger now, at 5 years old, but the pack still fits perfectly and there are still couple of inches of play in the straps), and the small goes up to 30".
Have you measured Alvin's girth? If you can wait a couple of months you'll have a better idea of what size to get unless he's well within the size range for whatever pack you decide on. Halo has the Ruffwear Approach and Keefer has the Ruffwear Palisades.