You're free to give as much money to you like, whenever you like, and to whomever you like without it affecting either your or their current tax position. Tax only becomes an issue if
(a) you die within 7 years of giving one or more gifts ;
(b) your estate is large enough for Inheritance Tax to be paid on it (which means above £325k, or above £650k if your spouse or civil partner pre-deceased you and left you everything from their estate ; and
(c) your gifts exceed the limits specified here:
https://www.gov.uk/inheritance-tax/gifts
So you're free to give up to £2,500 to your grandson for his wedding PLUS you can give away up to a total of £3000 to him or to him and to others in the year (or more if you didn't use up your £3000 allowance last year, as the balance can be carried over). As well as that, anyone who hasn't received money from you under those allowances can be given up to £250 (with no limit on the number of people that you make such gifts to).
So you shouldn't have any worries at all about what you're proposing to do.