Suggestion: find a business valuation professional, ideally one with demonstrable experience valuing businesses in your market, and ask the founder to agree to

Suggestion: find a business valuation professional, ideally one with demonstrable experience valuing businesses in your market, and ask the founder to agree to reimburse you for the cost of a valuation. The cost of the valuation might exceed the amount the founder has offered you, but if he is sufficiently motivated to acquire your shares and believes he is not significantly underestimating his company's value, he should be comfortable agreeing to such an arrangement. If he does not agree, it doesn't mean that your shares are worth significantly more than offered, but unless you need a couple grand, it's hard to see why you'd be motivated to sell.