It seems like he's eating a good range of food.
I would not worry that he's not getting enough variety. And even if he's limited in what he wants, he's eating pretty healthily.
Given that he has lost weight, and ruling out any health issues, keep in mind it might be just that he was very active or it might be a scale glitch, but also keep an eye for any other symptoms that he's not well. Also is he teething at all or had colds before he lost weight? Have a look at his chart in the long term, is his head circumference growing, is he growing in height at a relatively stable level. They are good signs he's doing OK. Also you say he looks healthy
I've been told by my health nurse and I agree (given that when I haven't bulked up DS's meals he doesn't gain weight) that I need to feed him up more than his natural appetite . We also just recently had a blood test to rule out celiac disease, which thankfully was a good result. It goes against what I encourage with my DD, but it's something I see as temporary measure.
The things I have tried... I add formula to the full fat cows milk in his cereal (I also used to add it and mix it well into spaghetti bolognaise). If he has a fairly low fat meal like pumpkin soup or baked beans, I mix in dry formula or cream cheese. If you think your DS will notice and refuse the food, start by adding it in in tiny amounts and build on it. I prefer good fats like olive oil, avocado and peanut butter. When he was off the bottom of the chart I added cream to cereal and olive oil to savoury meals as a temporary measure. I found the highest fat yoghurt (I discovered there are 10% fat ones available all the way through to no fat ones).
I give him peanut butter and cream cheese instead of vegemite on crackers. I encourage a big breakfast as he doesn't eat as well at bedtime (sounds the same as your LO). For me evidence that he eats enough in the day is that he doesn't wake up starving, he plays for a while. I give him meats, full fat cheese, and avocado.
Your LO might do well with food at his level he can pick at for a lunch meal or after nap, experiment with that. He might eat better if he has a small snack before a nap so he has a good appetite for al lunch after his nap.
I would think of the long term in adding more foods, and really concentrate on taking any pressure off.
For extra veggies I add grated zucchini and carrot into the spaghetti sauce and simmer it down. My DS still won't eat a lot of things my DD would, so one night if we have salad, I put it on his plate but don't expect him to eat it.
My DS had giardia at around 11 months and one of the symptoms is that you can feel nauseous when eating. To me I felt there was something wrong. I think he had it for about a month, getting constantly sick with everything else, and I was at the doctors once or twice a week until we got a diagnosis (after a paediatrician finished telling me he simply wasn't getting enough breast milk!!). He even dropped off the bottom of the chart for weight and it was so hard not to get really upset about it.
Nice to 'meet' another Melbournite
Come back, we're having a mini heat wave in spring
That's Melbourne!
PS I was impressed with these web pages relating to calorie boosters and picky toddlers.
http://www.askdrsears.com/html/3/T030800.asphttp://www.pediatrics.emory.edu/divisions/neonatology/dpc/calories.html