Hi M.,
Congrats on your little guy and on breastfeeding! Keep it up, it is the best food for him!
It sounds like he is fine.
I once heard a talk by Katherine Dettwiler (spelling? and her website is good , btw) on how there are many factors that affect the measurement of your baby's weight that have nothing to do with your baby's actual weight, just with differences between the two weighings at two different check-ups. For instance--was baby clothed or naked each time he was weighed? If clothed both times, was it the same type of outfit? If he wore a heavy outfit the first time and a light outfit the second, it would look like he hadn't gained so much. Same thing with a diaper--was it full (heavy) or empty (light) each time? Is he at the stage where he poops a whole bunch, once a week? If so, then where did the weighings fall in the "poop cycle"--just before, just after,or in-between? Was baby weighed on the same scale? when was it last calibrated? a lot can happen to a scale in two months. All these things could account for a significant change either way.
Solids won't hurt, he's about at the right age; check for the other signs of readiness too (can sit up unaided, doesn't push the food out of his mouth, showing interest in your food, that sort of thing). But as other moms have mentioned, don't quit nursing, that's still his primary source of nutrition.
Do you nurse him every 3 hours because that's what HE wants or because that's what YOU want? If it's him, that's fine. If he cries and you think he might be hungry, nurse him--if he eats a lot, he was hungry. Babies tend to stop when they have eaten enough!
Good luck to you and enjoy your baby!
K. Z.