Ooh, a rare disagreement: your "put option" scenario makes sense if there is an enforceable contract which obligates them to actually hire him in April. Would you be blocking off April for a consulting engagement in these circumstances?
This sounds a lot less like "contract signed and PO for pre-payment cut" and more like "strong interest in resuming discussions with you several months from now." That isn't a no-op in terms of negotiations, but you're well aware (and I learned the hard way, multiple times) that there is a very-much-nonzero probability that the discussion in April begins with "Well, startups move fast. Conditions have changed and we can't bring you on. [Optional: But we're really really interested in talking to you in July. (+)]"
+ Newbie consultant mistake which I'll confess to: this basically happened exactly when a potential client slow-rolled actually getting a contract signed for three months -- I need CEO approval for this, your point-of-contact turns out to be swamped in the engagement month so let's defer for a month and so we'll defer signing for a month, hang on the contract will be signed any week now but the CEO is busy, hang on we just hired someone and need to get him to speed on this before bringing you on, etc etc -- and only then expressed negative interest in an engagement, after the date had slipped multiple times. This, naturally, has downsides for the business, both because it consumed a lot of my time in stressful will-they-won't-they admin and because I blew several weeks of availability on the "Sorry, can't work with you in $YOUR_DESIRED_TIMEFRAME, it's already committed" basis.
Older-and-wiser me would have a) been more aggressive about getting them to commit to a contract (rather than a weekly or semi-weekly ping which practically oozed with "I don't really need you to take immediate action on this."), b) released the optimistic lock on the schedule the first time the client had any issue, and c) if that particular client were to contact me again, they'd get a two line email with a contract and wiring instructions for their 100% deposit attached, to assist them in efficiently administering their vendor relations.