Hey. I think if I had suspicions that my child was underweight this is what I would do:
1. Go to his doctor and have a consultation to check and see if his weight is so extremely off from "normal" that it warrants doing something about it. At around 7-9 months-old my son was about 10-11 kg. And, as I said, he is quite a muscular build and very tall for his age so, maybe 7.8 kg isn't that small for your son's particular build. I know you said you and your husband aren't tiny, which is true but I would definitely consider you both thin people. I referred to my dad and brother before--my dad, at his smallest/slimmest/fittest weight weighs 115 kg and my brother is similar. This is when my dad was in track and field in secondary school and winning in state competitions. I don't know what the size conversion for shoes is here in HK but in American shoe size my dad wears a size 15 and my brother a size 17...believe me, in comparison, you and your husband (and nearly every other person I know) is "slight" and "small." :0) So, those are the specific genetics I'm talking about when it comes to my son. I can't wait to see what my son looks like as a teenager (and uh...how much food he will consume??!!)
2. Based on what my doctor tells me, I would start designing meals that have higher calorie content. The amount (as in volume or bulk) of food probably isn't such an issue. A bowl of rice may fill your belly but if you're trying to put on weight it doesn't go too far toward that. So, I guess, maybe I would think about how I prepare the food--ways that I can add more healthy oils to the diet (i.e. cooking with more oil, giving foods that are higher in fat content etc.) If my son was found to be truly underweight, it's helpful to get more calories in his diet--so feeding more calorie dense foods would be an option. Also, there have been times when my son's appetite has seemed to be lagging--when it would get to the point where he was only eating one small meal a day and refusing other food, I actually fed him what is called "Chinese Appetizing Tea for Babies"--there are several brands but I bought mine at Watson's. It actually worked in increasing my son's appetite--I'd only have to use it for a day or so and his appetite would pick up.
3. Based also on what I find out from my doctor, I would add a baby vitamin supplement into my son's diet. I have one that I use with my son that's all natural. Sometimes I've found with my own body that it doesn't function as well without the proper nutrients--vitamins and minerals. Afterall, vitamins and minerals are really essential in helping all of us get the most out of the food we eat.
I really hope that you can pay a visit to your son's doctor and that he/she can point you in the right direction.