Every child is different. PLEASE do not compare your son to anyone else's baby. My 6 week old is 12-1/2 lb, and is gaining like a beast. My best friend's little girl is 6 months old and only 11-1/2 lb. However, they are both healthy and happy. The first thing I would recommend is to find a local La Leche League leader and talk to her about your problems. She will be able to go much more in-depth with you than we can online.
1. Milk supply - You may or may not know that milk production is a supply & demand system. The more your son nurses, the more milk your body will produce. Because he was getting supplemental bottles, your body was producing only enough milk for him to drink during the daytime. If you continue to nurse him on demand, your milk supply should quickly go up.
2. How much is he getting - The amount of time he nurses is NO indication of whether or not he is getting enough milk. My first daughter nursed for 40 minutes on each side, at least. After a while, I started cutting her off at 20 minutes, because of sore nipples. She was consistently top of the charts for weight until she started crawling. My second daughter nursed about 15 minutes per side. She was consistently top of the charts for weight until she started crawling. My third daughter is 6 weeks old. She nurses only 5-10 minutes per side most of the time. She has consistently been top of the charts for weight. Some babies are simply more efficient than others at getting milk out.
3. How often he feeds - Every child has a different pattern of nursing. You say 6 times during the day, so I'm guessing about every 3 hours? He may be trying to get your milk supply up, or he may be nursing for comfort. 6 months old is also a common time for a growth spurt, in which case he will nurse more often to increase your supply. The best thing you can do for your milk supply is to nurse on demand.
4. Eating solids - 6 months is the MINIMUM age recommended by the AAP to start a baby on solids. At that age, they are only playing with food; your breast milk should be his primary nutrition until he is about a year old. I know many babies who are completely uninterested in solids until a year or even 18 months. This is normal. In fact, some people speculate that it is a protective mechanism in babies who are susceptible to food allergies. They self-delay eating solid foods to give their bodies a better chance of not developing allergies.
5. Is he getting enough to eat - What do you think? Mom's gut feeling is often right. Is he happy most of the time, or fussy? Does his skin look nice? Is it a good color and does it have a plump feel to it, or does it look loose and saggy? If you gently pinch his skin, does it spring back into shape or does it stay droopy? Are his soft spots sucked way down? The best indicator of health is often just whether he seems healthy or not. Trust your mama instincts. Most of the time, they are right on.
6. But he's not gaining weight - Be aware that some babies have periods of time where they are not gaining weight. If he just gained a new physical skill like crawling or walking, he might even lose weight, or stay the same weight and keep growing taller. *Do not* worry about where your son falls on the chart. First of all, those charts were created based on formula-fed babies, not breast fed babies. The two gain weight totally differently. Secondly, those charts only tell you where your son is compared to the average population. By definition, someone must be in the 100% and someone must be in the 1%. That doesn't mean that your son is not healthy.
As far as the "little tube" that someone else mentioned, it is called a supplemental nursing system (SNS). It can be helpful if you need to supplement while you get your supply up. However, my friend has used it with both of her kids, and she would warn you that it is not an easy answer. She said it has a steep learning curve, and it will be several months before it is anything other than a pain in the rear to use. Also, it is the same problem as any supplementation - if your son is getting a certain amount of artificial milk, he will have less of a demand from the breast, therefore your body will make less milk. The SNS is better for your supply than giving bottles, but it is worse for your supply than continuing to nurse on demand.
I hope this helps, or at least give you some other ideas to think through. I really recommend contacting a local La Leche Leader. You can find a local group at http://www.llli.org/WebUS.html. You could have some mechanical issues like a poor latch that is keeping him from getting all the milk out that he could. More likely, your body is just making the amount of milk that he needed before he went off the bottle. With on-demand nursing, your milk supply is likely to go up. There are some supplements you can try to increase them - mother's milk tea, fenugreek, etc. You would do well to talk to someone who knows how to use those properly. If you don't have a local La Leche League group, you may be able to find help through the local hospital, or a private lactation consultant. You will likely have to pay for it in that case. I hope you guys get the help you need, and you regain your peace of mind.
Best of luck,