hi W.;
first of all, try to relax. you're a first time mom and we all go through this kind of phase; the more things change as the babies grow the more we realize we can't be prepared for all the unknowns, and we have to learn who our babies are as they evolve. you are doing a great job.
next; do NOT use formula. formula is NOT an alternative to solids; it's a 3rd rate source of nutrition that should really only be used in the event of breastfeeding compromise, if things really go badly. your breast milk is THE BEST FOOD YOUR BABY WILL EVER GET. solids are not better and he doesn't need them yet or any time soon. many nursers dont even touch solids till 12 mos or more. and anyway, think of the quality as this; formula is to breastmilk as Fritos are to a turkey sandwich on whole wheat bread with hummus, if you know what i mean.
more over, using formula and too many bottles could compromise your excellent and successful nursing relationship. so no matter what you ped says, and he sounds like a very nice, laid back person that you trust, stay away from the formula, it will only make things worse.
keep nursing your son as much as you can. many breastfed babies get interested in solids temporarily and then lose interest. as long as he is on the breast or getting breastmilk in the bottle while you are at work, he will really be fine.
also, the activity and intelligence level you describe in him, even while he's sick, is really great, and those kinds of things, plus wet and poopy diapers, good coloring, good sleeping, are what you look for to indicate good health over all. if something is really wrong, he's going to get lethargic and yellow and sad, and that's when you would worry.
leveling off in growth is also very normal for nursers; they often will plump up fast and early then slow down; (my daughter was a big fatty for a while, 25 lbs at 16 months, and hasn't gained at all and is now 21 months and getting tall and leaner.) one theory is that because breastmilk and breastfeeding help babies develop a higher intelligence and more intense personal contacts and interaction, they are livelier, more curious, interested babies, and they get busy during the day and will skip feedings. if you are really lucky on one level and he's a great sleeper, he may not be making up those feedings at night; but he WILL make it up. he will tell you when he's hungry, i absolutely promise. try offering him the breast as often as you possibly can when you're home, like every hour or two no matter what. this will help make sure he's remembering to eat when he's busy with the stereo and heavens knows what else!
as for crawling don't worry; my son didn't crawl at all till almost ten months, and then two days after his first birthday he got up and walked across the room and never stopped moving ever, since then; hes now 3.5. my daughter crawled a lot at 8 months but didn't walk until 13 months, and now she's 21 months and runs everywhere constantly. your son will get there, too.
lastly i would say if you really feel nervous go to your local La Leche League meeting; just google the organization and click on resources for your local chapter; even tho your nursing is going fine, they really are the most reliable, up to date resouce on infant nutrition. they may have a parenting agenda that is not perfect for you in totality, but that's fine! take what helps and leave the rest.
lotza luv,
J.