Well, yes, that makes sense.
As a joint executor of the estate, he needs to confirm his acceptance that the house will be sold such that the proceeds can eventually be shared between the four children. If he doesn't do so, the co-executor and the other children are liable to get ratty because it stops the division of the estate proceeding - until they strike him off from being an executor.
Just signing this doesn't mean that he is in imminent likelihood of receiving his share of the estate - it is when the executors divide the proceeds that he finally has his money.
Longer term, there is no easy way to stop this. Benefits are paid to those disadvantaged and with little money. If he comes into money, he can fend for himself until the money is consumed. If he won the lottery he would not surely expect to keep drawing benefits (I trust) - try him when that angle.
Personally I think he is just being greedy thinking he can receive both.