How do I tell him I actually hate the wedding venue we booked?

Sit him down tonight before he gets another word out about the paneer starters or the seating charts, and tell him you made a mistake on the lawn and cannot picture getting married under that red tent without feeling sick. You will lose money on the advance, and your families will be annoyed that you wasted a Saturday driving out there to look at catering options for nothing, but that financial loss is nothing compared to the weight of spending your wedding day wishing you were anywhere else. Do not wait until September or try to fix it with drapes and lighting because temporary cosmetic fixes will not change the sinking feeling in your chest. Tell him you need to lose the deposit and start looking at closed banquet halls or smaller indoor spaces this weekend. Expect him to be frustrated and stressed about the lost cash and the extra work it creates for both of you, and let him vent about that without getting defensive. Apologize for not speaking up before the booking was finalized while everyone was cheering, but hold your ground on the venue itself. Once the initial shock wears off, you can both look at alternatives that actually feel right to you.