Your baby is still gaining some weight - good.
Your nurse is not worried - good.
Your baby is happy and active - good.
Mummy is stressed - not good....
My DS was 4.3kg at birth and quickly dropped down the charts. He's still a lightweight but has been pretty steady at 40th-odd percentile for height. Have you looked at the WHO charts for a comparison - he might look better on those ones.

Are you bf as well as expressing? Are you working...? (sorry I get everyone's stories mixed up)
Teh best thing for supply is more feeding - so if you can get extra feeds in that would work best. Solids won't help with weight gains generally because they displace milk (best source fo nutrition at this age) and are generally fruit/veg etc at this point and so low calorie.

I don't think the day naps are any indication of low supply - they can be utter crap sleepers with plenty of milk in them.

The best indication of supply, besides weight gains, is output - how many wet & dirty nappies? (at this age maybe not so many dirty ones, but still getting lots of wet ones?)

I think you're doing very well and your DS is thriving on your milk - he's just not a big boy.