We could not recommend that you attend such an event since Lutheran churches do not have a valid priesthood and thus do not have a sacrament of the Holy Eucharist. While Lutherans who take communion in their churches in good faith may be blessed by God for their attempts to please him, in fact their community rejected the priesthood at the time they broke away from the Catholic Church, and so their ministers do not have the power validly to celebrate the Eucharist.
As a result, your attendance at the event in question would seem to imply that you recognize or endorse the idea that your grandson is actually receiving further Christian initiation by receiving the Eucharist (one of the sacraments of initiation). That is a message you must not send as it would constitute a form of false witness to your family and to your grandson.
Matters would be different if the sacrament were valid—for example, if he were being baptized or married in the Lutheran church (those two sacraments being valid among Lutherans) or if he were receiving Communion in a church that has a valid Eucharist (such as the Greek or Russian Orthodox churches).
But to attend a non-Catholic ceremony that one knows to be invalid—whether it be a baptismal ceremony, a marriage ceremony, or a first communion ceremony—would be to send the wrong message. It would be better, if more painful, to say, "We know that this is event is meaningful to you, but we cannot attend."