While I'm personally not a fan of unlimited pellets for adult pigs (mainly due to the fact that we have had so many issues with bladder stones and sludge over the years), I would agree that obesity isn't all that common in healthy, active guinea pigs on a good quality diet. Your guy is only 3 mos. and he's not fat. I would continue feeding as you have been for now, and start switching to timothy pellets once he reaches 6 months or so. At that point, you can decide whether or not to start cutting back on pellets. Again, I generally feed around 1/8 cup twice a day for adults, along with plenty of fresh veggies and unlimited hay, and none of our guys have been underweight. There is no hard and fast rule, though, and as others have said, mileage may vary.
If it were me, though, I would start cutting back on alfalfa hay now and start adding timothy hay to his diet.