Tiny baby struggling to gain weight - need advice

[Minieggmug]

First of all I know that nursery illness is normal and inevitable. Since my DS 12 months started a couple of months ago he has been ill pretty much every other week; we’ve had several colds, croup, tonsillitis and a tummy bug. Every time he gets ill he stops eating, sometimes for a week. He’s always been extremely small (2nd centile or less even though he was born at 40 weeks) and now whenever he gets ill his weight stalls or drops.

Is this normal and is there anything we can do? He’s so tiny he still wears 3-6 months clothes and I can’t see how he’s going to get bigger when he’s always off his food. He currently weighs about 17lb 5oz/just under 8kg.

What centile is he on now? Has he dropped since being ill or is it the same?

He’s now on the 3rd centile, when he was weighed at his 10 month check he was on the 6th.

My son is exact same as this, he is 23lb at 2 years and 7 months and very small for his age. He is constantly unwell from nursery and will stop eating completely for up to a week. The gp reassured me as long as he was drinking it was ok but this is also a worry for me