First question: does the life estate cover the whole acreage or just the home? If it's the home, then the loss is the capitalized rent. If it is the entire property, I would suggest the following:
Estimate the market value of the land and the home, and look at the underwriting tables for life expectancy, given the age of the man. That should give you a reasonable period for which the property cannot be used. Look carefully at any improvments. If you have a life expenctancy of 78 years, for example, that's a 25 year life. The gentleman who has a life estate is not going to maintain a home that he doesn't own beyond what it takes to keep it going. You can assume a minimal remaining value for the improvements. The value of the land at reversion can be estimated by using the Present Worth of $1 table. Look at long term bond rates, gaze in your crystal ball and estimate a 'safe' rate. Then use the Present Worth of $1 at X Years to apply a reversion factor. For example, if the land is worth $100,000 today and you forecast an overall safe rate of 7% over the 25 years to reversion, the factor is .016581. $100,000 x .016581 gives a current value for the land of $1658. As the estimated period to reversion decreases, the value to the property owner increases because the time to when the property can be used decreases.
In summary, given that the man has a life estate on the overall property and is relatively young, the value of the land to the person actually owning the property is very minimal.