My tech guy says we should use "HD-SDI coax" with converters. Can't we just use HDMI?

Short answer:
Your tech guy is right. HDMI is generally considered best for distances less than 25 feet; and HD-SDI is better for those longer distances.

Long answer:
It sounds like your tech guy knows his stuff well… HD-SDI is the industry standard for ”broadcast” style video signal cabling and connection along the longer distances commonly needed in professional projects. HDMI is considered to be a ”consumer” level standard. That is for home use, and a maximum cable length of around 25 feet.  So yes you would likely experience poor or nonexistent image at 150’ using HDMI. Having said that – you might find enhanced HDMI cable/systems that might work – but you may still end up with problems and or spend as much or more money than if you just invested in the HD-SDI converters and cable. You can get HDMI/HD-SDI converters for around $285 and possibly as low as $160 each. Another consideration is ”latency” or signal delay. While every thing introduces some ”latency” into the signal, HD-SDI typically has very low latency/delay,  but some other methods of extending HDMI past 50 feet will sometimes result in significantly noticeable visual delay – yuck!