Hi and welcome
Failure to thrive is generally recognized when a child drops 2 major percentile lines in a relatively short amount of time. Your son has crossed one line, so I see why your ped is not yet too concerned. It is very normal for toddlers to be picky.
I see an issue with a developmental delay though. Ask your ped for an evaluation by early intervention. At 18 months a full term baby should be taking their first steps, and if not it can be perfectly normal, but it warrants an evaluation to see if there is a delay.
Any time there are multiple food allergies you run a risk of FTT. Keep in mine though that around 18 months is when genetics start to take over and kids will begin to be large or small depending on who they are going to be genetically. If he continues to rapidly fall down his curve then there could be a problem, and if your ped still refuses to listen, find one who will.
As for the weight loss, I would be a bit concerned too. My DD is in a constant battle of gain/lose and all you can really do is 'calorie pack' everything he does eat. A nutritionist can help you with this. Good luck, hope to see you around more