Good luck finding a side by side 10 gauge period on those sites. They are rare.
One of the biggest questions to answer is what chamber length. The older 10 gauges will have a short chamber, under 3 inches. Many of the newer 10 gauges will have a 3 and 1/2 inch chamber. This makes a big difference in the weight of the gun as well as the shells you will use. Many modern 10 gauge side by sides are very heavy and to my mind they point like fence posts. Although for goose hunting you may already be thinking of getting him a gun with 3 and 1/2 inch chambers. Although I have not personally seen the big advantage. I prefer a gun that points quick and shoots straight over a poor handling gun that can shoot more pellets. But, hey, that is just me.
However, the key question is: does he load his own? 10 gauge shells are getting harder to find. Short 10 gauge shells even harder.
I had a modern low end Churchill side by side in 10 gauge. It handled like a roughly hewn chunk of wood.
