Hey Jo, just checking because I may be confused...isn't Caleb milk allergic? If so, then he can't have margarine...margarine has milk in it. I mean, double check your labels at least because perhaps you have a brand that is milk free, but unless it is labelled as a dairy-free margarine, then it's prob got milk powder in it.
Also, custard is made with milk, too. Perhaps the first time he just didn't react strongly enough that you noticed.
I think that when you have a milk allergic baby, quite often they have more fragile digestive systems, especially if they are still healing from the time that he was having dairy. Milk allergic babies are more prone to being allergic to other things and also may even just take longer to get used to a new food.
for example, gave bread wiht a tiny bit of margerine on it, so was it the bread or the marge?
I would suggest making sure that he only gets one new food every few days so that you can make sure you know what he is reacting to. I know that by 8-9 months a lot of parents tend to have moved away from the 3-day rule, but you have an allergy-prone baby and feeding can be quite different. It is important to go slowly - there is no need for a baby to be on complex foods or table foods, nutrition wise, by 8/9 months. Lumpy vegetables, finger foods made from fruits and veg are really okay. With grain products it might be best to stick to something that is easy to digest and that is more wholesome and "pure" than bread which has loads of additives, and can be very difficult to digest. Oatmeal or barley cereal, millet, quinoa, etc. He doesn't need complex foods, home prepared fruits, veggies, some simple one-grain cereals, perhaps think about introducing a small bit of plain meat or oily fish in the next few weeks?
Because Caleb is milk allergic (again, I am just recalling from your other posts, is he not on hypoallergenic formula?) then ALL dairy needs to be kept out of his diet - so that's any product that is dairy, contains any dairy or contains any traces of dairy - so things like margarine, which you don't think of as a dairy product, but does usually contain milk powder. Read all labels carefully and avoid anything with milk, milk powder, butter, whey, etc etc.