Trust for disabled person

Hi, I am advising a Trustee on a discretionary will trust and need some help with interpretation! The Will leaves 50% of the Residuary Estate on a discretionary trust with a disabled son and also 2 grandchildren (being “the other Beneficiaries”) as the beneficiaries. The Will (dated 2011) states:

"My Trustees shall hold the Trust Fund on the following trusts:

During the lifetime of my son to apply the capital of the Trust Fund for the benefit of my son and my other Beneficiaries as my Trustees think fit so long as not less than half of any capital applied is for the benefit of my son

to accumulate the whole or any part of the income arising from the Trust Fund and to apply any income not accumulated solely for the benefit of my son"

The Trustees want to exercise their discretion and pay 1/4 each of the Trust Fund now to the grandchildren. The remaining 50% would be held on Trust for the son. Can the Trustees do this given the clause stating that any income should be held for the benefit of the son? Does it mean in the whole of the Trust Fund or is it ok if the grandchildren get 1/4 each and then income is held for the son’s benefit in the remaining 50%

I hope that makes sense!