Blended family Issues...Fairness?

okay I am a married woman with 2 kids from a previous relationship a girl 11 and a boy 13, and my husband has 2 boys 13 and 14 from a previous relationship, my husband and I don't have much money but his ex does, so she showers my step kids with gifts and my kids are jealous, but let's get to the point, both my step sons and my bio son attend TKD school together, and this summer they all wanted to go to a Muay Thai camp in Thailand this summer, but my husband and I don't have enough to send my son (it costs $1000 dollars per person) but my husbands ex is going to pay for my step sons to go and will give an extra $1000 dollars just in case they need it, my son was mad and jealous that you could see it on his face, so I told my husband that it isn't fair that his kids get to go and mine don't and that he should tell them they can't my husband said that would be wrong to deprive them of a trip just because of jealousy, and said that his ex paid for it and she has custody (he gets them every other weekend) so he couldn't say anything, but my son has been very upset that his step brothers are going but he is not, I am upset for him because it is unfair that his step brothers always get to do things that he cannot because we don't have the money, and my step sons are excited and have been talking about it non stop and that makes it worse, what can I do to get my husband to see that it is not fair?