Certainly as he resigned as a councillor, he would not have any right to vote on this committee and it seems very strange to have a burial committee consisting of 3 councils. This seems to be to be an external body rather than a committee in the traditional sense and I can't find anything that discusses non-Councillor membership of external bodies....
The other consideration is that non-Councillors are not allowed to be appointed to committees that regulate and control the council's finances (LGA 1972, s102 (3)) and if it is a committee that sets the burial fees and therefore the income of the council, you could argue that he is not allowed to be a member. However, if he has just been appointed for the specific task of completing a grant, then there was no need to even co-opt him in my mind, he can help complete the grant application without the need to be a member.
The normal protocol is that membership is decided at the Annual Parish Council meeting in May and that membership is agreed by the full council - you could use that tack?