That's so strange that he spits everything else out! I agree with mashimaro that it could be tongue thrust, but you said he's been on solids for a month? Has he always been letting foods fall out, or is this a new thing?
You could try mixing rusks with his food, but I wonder if it's the flavour he likes or that he can feed it to himself? Have you tried spoon feeding him while he's eating the rusks? Dylan refused to eat off a spoon for a while, so I started giving him finger foods at meal time. While he was eating those I was able to spoon feed him some purées. Now your lo is obviously too young for finger foods, but maybe the same idea would work with his rusks?
Oh, and this weekend I mixed carrots and apple and ds LOVED it
Just another combo you might want to try
Let us know how you're doing!
Talk soon,
Vikki